Martin Jr. Nalugon

That is not a security problem just an inconvenience.
The only way that is possible is if the app she installed was malicious. Uninstall the app from you pc.What you are saying makes no sense.
 
Also this vv
 

What speeds are you getting from your ISP? if it is bothering you that much, set up QOS and limited the speeds on her device..

I don't know much about cybersecurity. But there isn't anything similar happening on your sister's PC because she is using all the apps ? Or didn't any of her friends face any other issues ? Seems like a Man in the middle attack or DoS ? .
 
If you are sure, about the attack, you can do one thing. Make an Account on Cisco Open DNS.  Force your router to Use Open DNS by setting DNS server in WAN to 208.67.222.222 and Alternate DNS Server to 208.67.220.220. Also install Open DNS updater app. After that before 12am night, shut off all other network devices, except your pc ( don't keep any app open ) . and the next day morning , open DNS will show you logs which Domains and websites were accessed.
 
If there are many, you can't block all, some are ad-related websites, (some are those windows update, time update, windows live tile updates) , so set the PC To metered connection temporary. Also if you block those ads, your web-page load times would get slow, because browsers load webpage - element by element , and they will wait for a longtime for ad to load, until being timed out. From OPEN DNS you will also get which domain was accessed the most - from there you can get a clue if it is a legitimate or not, and block that domain in opendns, so it stops further transmission of any data packets.
 
Currently I have blocked my router's one of its own server, because it access that domain 1400 times per day ! .. It is also from a Chinese company Tenda.
Once you block a domain, and if you manually access it from browser, you will see something like this 

*UPDATE* 
 
sorry a bit late, its 12:40 AM as of 28/09/2020 and I also reinstalled windows twice to make sure that there isn't anymore viruses (because my sister used it in a jiffy with her app and we will get into it a bit)
 
basically all of these problems were not caused by ip conflicts or faulty hardware
 
i checked everything from pinging them from in cmd to see if there were any loss
ping 8.8.8.8 -t  ping 127.0.0.1 and pinging my router's ip address   
So my sister's app is causing the network to crash which was a Spoof attack from according to my brother and explains on to why my Instagram got hijacked last week. My brother launched a program called Xarp which they use for cybersecurity (probably)  from his past job.
 
The application or program is either Acadsoc or ClassIn (ClassIn is a virtual classroom like Zoom) and these are the 2 apps that is used by my sister for her teaching students from her agency which is Acadsoc and is requiring ClassIn as their go to program.
 
My brother did suggest buying a better router with a built-in firewall but I said to him to hold on to it because it is going to be a joint project by the family.
 
All I can say that my connection is getting spoof attacks which explains all of those downtimes and also "Ethernet Connected but no Internet Connection" problem that I am posting to this thread.
 
@Aditya Dighe Thank you for your advices :'>>> they helped a ton and also @LAwLz for boosting my moral when this kept happening and to all the people that helped on this thread.
 
If there are any recommendations and procedures you guys want to help and say here in order to prevent spoofing will be really appreciated ahahhahah, I'm sorry if I sound a bit demanding but I really don't have a single clue in cybersecurity. 
 
 

*UPDATE* 
 
sorry a bit late, its 12:40 AM as of 28/09/2020 and I also reinstalled windows twice to make sure that there isn't anymore viruses (because my sister used it in a jiffy with her app and we will get into it a bit)
 
basically all of these problems were not caused by ip conflicts or faulty hardware
 
i checked everything from pinging them from in cmd to see if there were any loss
ping 8.8.8.8 -t  ping 127.0.0.1 and pinging my router's ip address   
So my sister's app is causing the network to crash which was a Spoof attack from according to my brother and explains on to why my Instagram got hijacked last week. My brother launched a program called Xarp which they use for cybersecurity (probably)  from his past job.
 
The application or program is either Acadsoc or ClassIn (ClassIn is a virtual classroom like Zoom) and these are the 2 apps that is used by my sister for her teaching students from her agency which is Acadsoc and is requiring ClassIn as their go to program.
 
My brother did suggest buying a better router with a built-in firewall but I said to him to hold on to it because it is going to be a joint project by the family.
 
All I can say that my connection is getting spoof attacks which explains all of those downtimes and also "Ethernet Connected but no Internet Connection" problem that I am posting to this thread.
 
@Aditya Dighe Thank you for your advices :'>>> they helped a ton and also @LAwLz for boosting my moral when this kept happening and to all the people that helped on this thread.
 
If there are any recommendations and procedures you guys want to help and say here in order to prevent spoofing will be really appreciated ahahhahah, I'm sorry if I sound a bit demanding but I really don't have a single clue in cybersecurity. 
 
 

Generally reinstalling helps anyway, antivirus could also cause issues.  Pretty much everything on your system could, that's the thing about Windows it's enormously complex, and you can't really have a granular control or view of anything because it's all just kinda jumbled together.  At least from my experience anyway. 

I wouldn't worry about DNS attacks on a local secured network, that's more of an issue that happens to public websites and perhaps your ISP. 
 
I know some of these may have already been covered, but just some things to think about if your reinstall doesn't fix your issue: 
When your internet cuts can you ping other devices on your local network - can you ping your default gateway (hopefully your router) when your internet cuts?  Have you checked Event Viewer (you can search for it in start), check out the System and Application logs after your internet cuts.   Look through it and report through any driver issues, or any reported issues with the network.  - usually the cause of the issue won't be the event itself but events around the event.   
If for some reason you are infected with malware, which is pretty unlikely if you have good antivirus (most targeted attacks happen to organizations and governments) then reinstalling should definitely resolve your issue. 
 
My best guess is it's a driver issue, either way, check your event logs if you get another cut out and send us through anything of concern.  It will give us more information about what's going on. 

There is Safe mode for that using advanced startup.
 
I know that also won't 100% guarantee you that a specific program is not causing a problem, but many people have many important apps like office, photoshop, Adobe Premiere Pro, which need to be activated. Some settings need to be done again. Although this more applies to an enterprise PC and just for consumer PC doing a complete fresh install is more easy, other backups also need to be taken, of C drive ( desktop, documents, etc) ....  That is why the refresh was introduced. 
 
Also then there is another type of upgrade - which installs the same version of windows (if you have the setup) just without the apps. And then it is the Complete wipe and install. If he personally has tried so many anti-virus apps, I don't think there is a virus. Personally - I don't install any antivirus app, Windows Security does its jobs well, others are a bit burden to the PC (especially a bit older ones) . System Restore is also there, if just a newly installed app causes an issue
 
The problem could be due to multiple reasons
 
Out of which we ruled out - Router, Cable and connector - because his sister's PC works just fine, no connection time outs in ping, even when he connects the LAN Cable connected to his PC to his sister's PC.
 
Now it Could be PC/ Drivers/Windows/Registry , Ethernet add-in card, or that particular RJ45 socket. He seems to have tried sfc scannow and DISM , but these commands always don't guarantee the working.

Yea okay, then you have to go ahead with reinstall. Or rather just refresh your PC, instead of entirely reinstalling ? It is easy too, Your Programs remain same. Just download Windows, open setup.exe . Or do it form advanced options.

It worked for a day and suddenly disconnects.
 
I decided to clean install my Windows because I think my OS kinda got infected by some virus when my sister plugged in a USB with malware. I did scanned them with Malwarebytes, Avast, and also my brother suggested which is BitDefender and there were none.
 
Thank you for all of the replies and wish me luck in a few days because my brother will be clean installing Windows back into my PC for literally cleaning the PC also. 

It worked for a day and suddenly disconnects.
 
I decided to clean install my Windows because I think my OS kinda got infected by some virus when my sister plugged in a USB with malware. I did scanned them with Malwarebytes, Avast, and also my brother suggested which is BitDefender and there were none.
 
Thank you for all of the replies and wish me luck in a few days because my brother will be clean installing Windows back into my PC for literally cleaning the PC also. 

So when you have that issue are you able to access your router's settings page or transfer files between yours and sister's computer ? So we can be sure about ethernet being working. If yes then we can also be sure about PC related issue as @LAwLz said. Do a DISM restore and sfc scannow from cmd admin and see if it solves your issue. Or we can try updating drivers
 
Else then the DNS the pc is reffering to will be router's DNS. 

Doesn't sound like an IP-conflict at all to me. Why do you believe it is?
 
I don't believe there is anything wrong with your router either, unlike what @Aditya Dighe seems to believe. If it was a router problem for example overheating or low memory then it would affect all devices on the network, not just your PC specifically. 
 
If it works for everyone except your PC then the issue is with your PC (hardware or software) or possibly some cable/port connected to your PC.

currently its working but I am anxious that it might come back so that's kind of a bummer for me if it does come back again and had to do some things for this. I'll buy a usb to ethernet adapter because for some reason. Thank you for the replies :'>>>> I'm literally suffering on how it will be doing in the next days
 

Just update your firmware once , https://www.tp-link.com/in/support/download/tl-wr940n/#Firmware
 
choose hardware version above.

Fair enough, I also use a PC from 2014 and my gigabit ethernet port just works fine. You shall have no problem with it.  It is just a router/cable/connector issue 

Just check the cables and connectors once, do clean them . And if there are any sharp bends on the cable, just straighten them up. Windows doesn't detect those minor disconnections from the internet
 
Reboot Router from Router Web Page once in a day , if there is an automatic option ,set it at some at night like 3 A.M. , so some memory gets freed (especially the memory it uses to keep logs ) 
 
Set your Wifi Channel different from your neighbours. 
 
And You can Static IPs, no issues, but just a better solution is to use DHCP and set lease time to 1440 minutes ( 1day ) or 86400 seconds ( 1day )
 
And Most importantly - Just update the router firmware if there is one available. It could solve most of your problems. 
 
 

@Martin Jr. Nalugon If your getting a new Router, look at the ASUS range.

USB to Ethernet adapters work great.
Most often there is a problem with ip configuration, router trying to assign same address to multiple devices.
You can configure a static IP in the right range and see if that makes a difference.

Or disable IPv6, maybe it's causing trouble...

Amazon has an Ethernet card for £9....
https://www.amazon.co.uk/TP-LINK-TG-3468-Gigabit-Express-Network/dp/B003CFATNI/ref=sr_1_3?crid=3JQ33G9XYIBTY&dchild=1&keywords=pci+ethernet+card&qid=1600614453&sprefix=PCI+E+etherne%2Caps%2C469&sr=8-3
 
Or
USB to Ethernet for £10......
https://www.amazon.co.uk/Rankie-Network-Adapter-Gigabit-Black/dp/B010SEARPU/ref=sr_1_3_mod_primary_lightning_deal?dchild=1&keywords=USB+to+ethernet&qid=1600614629&sbo=Tc8eqSFhUl4VwMzbE4fw%2Fw%3D%3D&smid=AB4RD1JXHRRE1&sr=8-3