Malaber

Uhh. I know my way around windows but I can't help you with that particular router. I know of them, but haven't ever used one myself. I do believe it's elaborate enough to have these functions. Windows doesn't have may options either. They're being pretty stupid about networking. Home doesn't have the functions and Pro assumes you're a friggin engineer that inputs coffee and outputs ones and zeroes. I'll try and explain so you can help yourself.
 
First off, the network settings in Windows 10 can be found in Control Panel\Network and Internet\Network Connections. Right-click on a device and pick Properties to modify the settings.
The built in firewall in Windows is pretty awful. You can't make a rule in it based on the hardware. It could solve your problems if you could. But meh.
So, QoS (Quality of Service) is a protocol that makes the hardware do it's very best to guarantee connection for specified devices. You can turn it on and off in Windows Network settings but you should also look though the router settings to enable it there too. On for everyone else and off for you.
For you to be able to set settings (like deny connection) that apply only to your computer, you must either use the MAC address or the IP. Your MAC address is kind of a serial number for the network device. The router may or may not be able to use it in settings. If not, IP it is. You have to set a static (one that doesn't change) IP address for your computer and then in the router firewall/NAT settings find a way to deny access for that particular IP. You should know that if you have one static and several dynamic (ones that do change) you risk some other computer randomly getting the same IP. That's why you might need to set static IP addresses for all the computers in the house. 
LAn is Local Area Network and WAN is Wide Area Network, also known as the Internet. It may or may not be the case that once you deny access for the IP, it denies for both LAN and WAN. That'd be a problem. 
Lastly/Tl;Dr, NetLimiter is a third party (made by someone other than yourself or Microsoft) firewall program I personally use. It's a lot easier to use. You can also use it to get stats on your usage and so on. But most importantly, it allows you to set firewall rules based on hardware. Simply create a rule that denies access to Internet but allows to local network for the old connection and no rules for the new connection. 

Sorry, man! I can't quit the game I'm in just yet and I got to be quick about this. There's lots you can do. Turn on QoS for everyone else, but you, use a program like NetLimiter (it's free). Maybe you can set a static IP for your computer in the old connection and block it in the router's firewall. Albeit, a lot of routers don't have separate network rules for LAN and WAN, but if yours does, I'd definitely try this. If yousing IP isn't possible, maybe you can use the MAC address.

No, I got it. I really did mean to suggest that you leave everything as is. Both connections enabled and used automatically. Windows will detect that you get better ping over the new connection and only use it for Internet. Windows will also notice that the NAS or the printer are not present in the new connection and only use the old one to connect to them. All this happens automatically by default. Are you experiencing problems with this? For example, does the old connection lag for others if you play? Can you see in the task manager that your computer isn't using the new connection for Internet? (Have the Performance tab open and run speedtest.net. You'll see a distinct bump in the connection it uses.)
 
If it works, don't fix it.  

Are you experiencing a problem with this? I mean, Windows should automatically detect the better connection and use it. So it'd only use the old connection for Internet if the new is malfunctioning somehow. And obviously it's never going to use the new one for NAS or the printer since the IPs for them are in the other subnet.

The Problem is already solved But yes 6Mbps is the fastest Internet connection I can get around here... We will get glassfibre in a year, but until then I'll have to live with 700kbs download I guess