Syncing television with apple tv

With your TV remote, open its own Settings and look for the HDMI-CEC setting and make sure it is turned on. Different TV manufacturer's call the setting different things.

With the Apple TV remote, go into the Apple TV Settings-> Remotes and Devices, Control TVs and Receivers. Make sure its on. Toggle it off and on if its already on. link-> Control your TV or receiver with your Siri Remote or Apple TV Remote - Apple Support

Then try again.

That is a function os HDMI CEC and is dependent on your TV. If it is not working as intended then check if the TV has any firmware update, refer to the manual and contact the manufacturer if necessary.