See here: If your Siri Remote or Apple TV Remote isn't working - Apple Support
- Check the battery charge.
- Restart just the remote.
- Pair the remote again.
Also try disconnecting the HDMI cable(s) on both ends for a while, to reset the HDMI connection.
Also check that your settings haven’t changed for this functionality, that HDMI-CEC is turned on for all HDMI devices.
On Apple TV: Settings﹥Remotes and Devices﹥Control TVs and Receivers﹥On
On your TV: See its manual.
Apple TV 4K, tvOS 15
Users are advised to keep their devices up-to-date, for best support, new features, stability, and fixes.
Update your Apple TV - Apple Support
About Apple TV 4K and Apple TV HD software updates - Apple Support