Problem solved by replacing the Archer C6 router with no-name one which works straight out of box as expected.

I've put the C6 one meter apart from fuse box&power outlet where it was before, connecting it to another power outlet. So proximity issue doesn't matter now. Disconnected all wired connections....

@Kevin_Z Thanks for the reply. It seems that the proximity of the router to the power outlet and/or fuse box is an issue. I did nothing else other than opened the box door onto the C6 is attached. It...

Hello, my Archer C6 is randomly dropping fixed (LAN) and wireless (WIFI both 2.4 and 5GHz) connections on PCs (three PCs in perfectly fine condition). WIFI connection on mobile devices (both 2.4 and...