Apple TV (both HD and 4K) refuse to connect to HomePod (2nd Generation)

I am reaching my limit of patience here, having tried to sort it out with a member of Apple Support to no avail (have been waiting for two days now on a follow up).

I upgraded from a HomePod mini a HomePod 2nd Gen a couple of weeks ago and intended to use it to pair with my Apple TV HD. It worked fine for a couple of weeks and then it suddenly stopped (our internet supplier went down and the problem emerged after that). The issue was that Apple TV HD refused to recognise that there was a HomePod when I tried to manually force it to AirPlay audio to the Apple TV HD.

After trying multiple restarts, forgetting the wifi network, factory rests on both the Apple TV and HomePod, and resetting my TPLink Deco Mesh home network system twice, I caved and bought an Apple TV 4K, thinking it the problem was with the older Apple TV HD.

I put the Apple TV 4K out of the box, set it up, and selected the HomePod 2nd Gen as the default audio. While now the Apple TV 4K can "see" the HomePod, it says it cannot connect after a couple minutes of trying.

As far as the Home System, both devices are set to the same room.

Both are connected to the same network, which is structured through a TP Link Deco Mesh system, with a main hub connected via ethernet to the Reuter, which is the unit the Apple TV 4K and HomePod are both connected (there are a further two Deco Mesh satellites elsewhere in my house).

I strongly suspect the issue lies in the devices not being able to "communicate" due to some unseen way that the WiFi network operates. But regardless, given how common Mesh systems are in houses, this should not be a problem.

Has anyone managed to sort this out? I am getting very close to boxing all the HomePods and the new Apple TV that arrived today into a box and buying a soundbar.