I lost the ability to turn on and off the TV
This would rely on HDMI-CEC, so verify that both devices still have this function activated:
- on Apple TV: Settings﹥Remotes and Devices﹥Control TVs and Receivers﹥On
- on TV: See its manual. CEC may have a different trade name per brand TV.
Then setting the Apple TV box to sleep mode, would also send commands to set other HDMI devices to standby, if they support such command from other devices. So it would be indirect through the Apple TV box.
A while later the volume function stopped working.
See: If the volume buttons on your Apple TV remote aren't working - Apple Support
A restart of just the remote can often bring the functionality back.
Else configure new custom IR settings for the Siri Remote to mimic these from the TV remote.
Control your TV and volume with the Siri Remote - Apple Support