@Cyprus_Phil Your observation is correct. When in AP mode the RE can't use its mesh feature. But you can always set the RE (in AP mode) to have the same SSID as your mesh network. Another approach...

@Spawn_e_git At the moment probably the best approach would be to use another AX1800 inside a plastic box. You may even skip the box drilling for the antennas - plastic won't work as a Faraday cage....

@Frank-Luffy_64 You can compare the RE models by their Wi-Fi speed or othe rparametes - here. RE315 or RE450 would be a good candidates but the final choice is yours. In this thread you can see a...

@Sreener Q3.4: Can Powerline adapters work if they are separated by different electric circuits? A: No. If they can pair in the same room, but the powerline LED turns off when you move one powerline...

@Frank-Luffy_64 Here's a guide on that. The difference would be that you have to select the EAP wireless SSID instead.

@Jordo EAP225 is an enteprise access point. Each one should be connected to your main router or a switch connected to your main router. Daisy-chaining these is not possible - details.