Ethernet not working with three routers

[Sadistic_Potato]

My father has three routers in his restaurant to cover the large area with sufficient internet, and the ethernet is acting very strangely as of late.

 

So the main router is where the internet actually comes in to, and that router provides further connection to routers 2 and 3. So the connection is ISP -> 1; 1 -> 2; 1 -> 3. Previously, both the WiFi and Ethernet worked just fine, but lately the ethernet connection has dropped for no reason. Apparently, it came back on a few days back, then went down again. 

 

What's the problem? He checked all the cable configurations and everything is connected properly. Since this setup worked fine a few months back, why is it not working when nothing has been modified? 

 

Need help ASAP, and will appreciate all help. Thanks. 

[Tedster]

I'd run a cable tester on each cable.

[Sadistic_Potato]

I'd run a cable tester on each cable.

 

The cables are fine. All connections, setups, and actual cables have been thoroughly with no avail. I'm rather curious as of why a perfectly fine setup stopped working for no reason, decided to fire up again for no reason for only one day, and then shut down again.  

[dzonidev]

The cables are fine. All connections, setups, and actual cables have been thoroughly with no avail. I'm rather curious as of why a perfectly fine setup stopped working for no reason, decided to fire up again for no reason for only one day, and then shut down again.  

 

There might be someone else other than the guests is on the network like neighbours... Also, what routers are you using?