@MackPhoenix62 may be too late but something worked for me so I thought I'd share it. I was using the X20 and it was connecting to the main unit which was far and resulted in weak signal even though there was another perfectly good unit in between them. I did the following to fix it:

1- reset the x20 unit that had the issue 

2- moved it further away so it had no way of detecting the signal from the main unit

3- set it up by adding it as new unit to the deco network (make sure to remove it from

the network in the deco app first). That is part of step 1 

if it connects to the main unit again, just keep moving it further and it will eventually restart automatically with red blinking light and connect to the correct unit 

  @TGC68 

So, it would seem that the latest version of the Deco app (Android at least) has fixed the issue with my M9 devices not connection the nearest Deco 🤔

I did miss a post relating specifically to M9 BETA firmware upgrades but have decided not to install it - weird that the BETA was available in Jan 2021 but 'official' version has still to be released almost 2 years later!!

Here's a link to the post for anyone that might find it useful - https://community.tp-link.com/en/home/forum/topic/215924?replyId=553404

  Yikes this is still a massive problem up until now. I have this exact issue with the X68. Looks like they only fixed it for the M series