If your Apple TV remote (Siri Remote) isn't controlling your TV or soundbar volume anymore, follow these steps to re-sync or reset it:
Press and hold the TV/Control Center button (the monitor icon) and the Volume Down button simultaneously for about 5 seconds.
The status light on the Apple TV box will turn off and then back on.
Release the buttons and wait 10–15 seconds. A "Remote Disconnected" notification will appear, followed shortly by "Remote Connected."
Test the volume buttons again.