AppleTV (4K) no longer connecting to wifi preferred network

I have an Australian NBN reseller provided router which I then wirelessly connect to an Eero 'mesh' network of 4 devices throughout my house. From lights to smart plugs, Apple laptops and iPhones I have about 40 wifi devices connected to Eero without issue. I do not use the NBN router wireless network for anything.

About a week ago my 64gb 4K AppleTV decided it could no longer connect to the internet despite being connected to the internet (in Settings, connected to Eero network, 5 bars (dots), looks correct). It has been connected to this network for about 12 months now, never an issue.

I have restarted the AppleTV multiple times now, no change, Connected but Not Connected. Can't stream anything off the internet.

If I change to the NBN router, it can stream from the internet, even though I now only have 4 dots instead of 5.

If I change back to my Eero network, the AppleTV cannot connect to the internet. It can however connect to Home Sharing from my Mac Mini and see anything I have in my iTunes (Music/TV) library.

If (while connected to Eero) I then check to confirm the Network settings, I get the little animated spinning 'wheel' that spins and spins until I have to hold down the Menu button to return to the normal AppleTV interface. Or wait about 10 mins and it eventually returns to the Settings, Network screen.

I discovered I was still on iOS 14.6 (auto update has always been on), but when checking for any software updates (whilst connected to the NBN router), it installed iOS 16 (whatever happened to iOS 15?).

Updating to iOS 16 has made no difference, still cannot connect to the internet when connected to Eero.

Until the issue last week, AppleTV is used daily with no issue.

If I stay on the NBN network, my other devices (laptops, iPhones) cannot Airplay to the AppleTV so this is not a solution.

Can anyone suggest what the problem might be and how to resolve? Everything was okay until a week ago and I am not aware of any specific event that occurred that may have caused this.