Apple TV 4K 3rd generation not compatible with Hue ?

… The app states I must us a 4th or later generation Apple TV. This makes no sense as there is no newer version available on the net.

The 2015 HD model was originally named Apple TV (4th generation). With the 4K models, Apple started numbering from 1 again (confusingly and needlessly imho).

2007 – Apple TV (1st generation) “A1218”

2010 – Apple TV (2nd generation) “A1378”

2012 – Apple TV (3rd generation) “A1427, A1469”

2015 – Apple TV (4th generation) / Apple TV HD “A1625”

2017 – Apple TV 4K (1st generation) “A1842”

2021 – Apple TV 4K (2nd generation) “A2169”

2022 – Apple TV 4K (3rd generation) “A2737, A2843”

It should be compatible to use as a HomeKit Hub. Are you sure you have the latest ATV 4K?

Identify your Apple TV model and it should be running tvOS 18.6.

What model is the device you’re viewing the Home App on? And what software version are they on? Look in settings> General > About. Or a Mac running older software also will not work.

Once you had upgraded to the new Home architecture, iPads could no longer be used as Hubs and any iPad or iPhone running older software, the Home App will no longer work. If I recall you need at least iOS 16.2 or later. iOS 15 or iPadOS 15 will not work at all.

It seems that the remote can turn ON/OFF the TV itself, the Apple TV light lights up, so it is connected. But none of the other remote buttons seem to be working.

Try pairing the remote again. Without the proper bluetooth connection, some remote activity may still be detected, even when not responding to it.

If your Siri Remote or Apple TV Remote isn't working - Apple Support

And I just read something about replacing the battery. Does anyone know how to do that?

The battery in the Siri Remote is not replaceable. If the battery is no longer good, then the whole remote needs to be replaced. I don’t think that is the issue, though.

Only in the old style remotes, originally bundled with the legacy A1469 and earlier, was the non-rechargeable battery replaceable [2].