Do not update yet!!!! Let's see if we can get you going first, the newer versions have a bug for the Atmos.
[Edit: posted the reply below before seeing that Pup had said more or less the same thing.]
The thing is, I'm pretty sure you need tvOS 14.6 or higher to play Spatial Audio. (@popshop's new rig only has 14.4 out of the box.) The bug was introduced in the update from 14.7 to 15.0, as I recall.
Here are the instructions to upgrade.
https://support.apple.com/en-us/HT202716