@PKP You wrote: "Ping from 192.168.2.* to internet works with TP Link Archer C2, but not with Archer C6. Same configuration." I really can't say why's that. It needs some troubleshooting. Try these...

@Kreco Check this answer.

@ukmartian You'll know that when you set the same SSID and password and observe if the plugs are re-connecting successfully. If they don't, then you have to re-add them, because depending on the...

@the_potato There shouldn't be any interference since it's the same transceiver for guest and regular Wi-Fi, just different SSIDs.

@PKP Are you sure about the router 2 ? AirGrid M5 HP is an Ubiquiti wireless access point CPE.

@ukmartian Judging by its web interface you can't use a separate SSIDs for the 2.4 and 5 GHz bands: