This is the EU hardware model.

I have recently installed the newest firmware that supports easymesh and wired backhaul on the axe5400 - so went down to the shop and got a re450 to try it out for a part of my house that doesn't normally get wifi. The re450 is in another room connected through twin power line adapters (PA71010P v1 on the 2017 firmware update).

After successful wireless pairing in a closer room, the home network falls to pieces as soon as it's moved to the other location that is connected via the powerline adapters.

Devices constantly drop in and out on wifi and attempting to access the router using the tether app can't find it either when this happens - even when using web only rather than through wifi. No error lights or similar seem to show on the router.

Then, as soon as you unplug the extender or put it in a closer room so it is using wifi rather than Ethernet backhaul through the powerline adapters, everything goes back to working normally again. When the system is online, the Ethernet symbol shows up on the web app with all blue lights on the extender so the Ethernet backhaul does appear to be working (if it didn't, it wouldn't be able to connect to the router due to range regardless).

Has this been encountered before/any suggestions on how to address?

I have tried updating all devices to latest firmware, factory reset them all a couple of different times etc, went with sharing the 2.4ghz/5ghz ssid and splitting it, the result is the same. I will try to plug it via direct ethernet from the router to see if that works and update this too.

I wondered if it might be a similar situation to this, but i updated the firmware on the powerline adapters and factory reset them, re-paired and the issue persisted:

https://community.tp-link.com/en/home/forum/topic/620116?replyId=1245044

Edit: Have now done direct ethernet to the adapter and it seems to work fine, giving the desired ethernet symbol with no dropouts, so presumably this is something to do with the powerline adapters causing havoc. Note: In that room, I have a desktop computer which has worked fine in this configuration for a number of years.