@Banjopatty Yes, that's correct, they both do support an Easymesh ethernet backhaul - details.

@grace1122 The default LAN IP address of the WR840 is 192.168.0.1. Try to open the http://tplinkwifi.net when you're connected to the WR840. Here's a video on how to set it up in RE mode.

@Banjopatty Putting a PLC adapters or REs usually is used when there's no cabling options. Cable connection will give you the best bandwidth and stability. In your case (AX20) can be connected to...

@PrasannaBJN Press the reset button for about 10 second to reset your RE : Then reconfigure from scratch.

@zageng According to the screeshot your 5 Ghz band setup is fine. What exactly is your issue ? You may update your firmware to the latest one from here (direct link).

@Gamool Check here.