Just an update.

Deleted the network. Reset all devices (x73-dsl, x20- DSL, 2 X x20).  

Plugged each one in separately when creating new network starting with x73-dsl as wi-fi router. Every other unit took a few attempts to be seen when adding to the network.

Almost completely lost wi-fi connection in the next room. Extremely slow. Using Android phone and showing in app as connected with strong signal to 5ghz. Whole network intermittently dropping for varying periods of a few seconds, never quite reached a full minute. All units still showing as green during connection difficulties.

Swapped x20-dsl in as main Deco. Coverage throughout house improved. X73-dsl still intermittently not being seen in app (light still green). Disconnected from all ethernet connections. Unit seems a bit more stable with nothing connected to it.

So, anyone have any other ideas for me to try?

  @David-TP 

Hello, I may have already advised this via email but...

1. ISP is Talktalk 2. unit showing as can't be found by the app normally for short periods of time before being found again 3. Normally only about 12 online clients at any time, no smart home devices as yet. 4. All devices affected. Fast roaming not used and mesh turned off for items that don't move around the house (speakers, PS4, etc.)

Replacement unit received, installed as satellite only and started doing same thing. Debug firmware installed and stability marginally improved, but network still affected. Doesn't matter if straight wi-fi or ethernet backhaul used. No unit EVER stops showing as green. Wi-fi access affected only, ethernet connections still work fine.

I've removed the X73-DSL again due to this problem and can only think at this point to return it for a refund.

I'll wait a few days for a response as it's perfect to get it working, but it needs to be stable and have a suitable range.

Regards, Bill