I have an Archer C8 connected to my modem/access point. via powerline adapter, DHCP is disabled on Archer, when i plug the cable in to the lan port the internet works on Archer but its very unstable, and all the smart features on the Archer C8 don't work i cant use the TP-Link Tether app even.

how i can solve that?

also as my modem works as access point if i disabled the wireless on my modem and connect 2 routers to the modem, and disable DHCP on the modem and leave it on in both router will sort this problem?