Lost connection from access point to main router

[hlongsoft]

Hello everyone.
I have problem with my home network.
I have 2 routers:
- Main router (modem, fiber converter) from ISP
- D-Link Dir 300
Main router uses 2 LAN ports: 1 to my 1st PC and 1 to D-Link router.
D-Link router uses 2 LAN ports: 1 to my 2nd PC and 1 from main router (above).
I set D-Link router to Access Point mode, disable DHCP, NAT.
Everything was just fine until several months ago.
My 2nd PC usually disconnect from the Internet. While using normally, the network icon suddenly changes into warning state. Cannot connect to the main router (I use ping command to test).
Other devices connect to the Wifi have the same problem.
When problem occurs, I need to unplug the network cable between main router and D-Link router then plug in again to solve the problem (or restart the D-Link router).
Meanwhile, my 1st PC can still connect to the internet normally.
I tried to replace the D-Link router with another router but the problem is still there.
I tried to divide the network into 2 networks (use WAN port on D-Link router) but this problem still happends.
I don't know what's going on here. Sometimes, it works for a long time without any problem.
Please help me. Thank you very much.

 

[bill001g]

Since you replaced the router the only thing really left is the ethernet cable. Generally the do not fail intermittently but there is not much else to try.

 

[hlongsoft]

I replaced 2 jacks of the cable too but have not solved the problem