Hisense TV & Apple Remote

Here are some things you can try to get the Apple TV remote to control the volume on your Hisense 4K UHD TV:

1. Make sure that HDMI-CEC is enabled on both your Apple TV and your TV. HDMI-CEC is a feature that allows devices connected to your TV to communicate with each other. To enable HDMI-CEC on your Apple TV, go to Settings > Remotes and Devices > Control TVs and Receivers > HDMI-CEC and make sure that the switch is turned on. To enable HDMI-CEC on your TV, consult your TV's manual.
2. Try learning the volume control signals from your TV. If HDMI-CEC is enabled and the Apple TV remote is still not controlling the volume, you can try learning the volume control signals from your TV. To do this, go to Settings > Remotes and Devices > Volume Control > Learn new device. Point the Apple TV remote at your TV and press the volume up or down buttons. The Apple TV remote should learn the volume control signals from your TV.
3. Reset your Apple TV. If learning the volume control signals from your TV doesn't work, you can try resetting your Apple TV. To do this, hold down the Menu and - buttons on the Apple TV remote for six seconds. The Apple TV will reset and then restart.
4. Contact Apple Support. If you've tried all of the above and the Apple TV remote still isn't controlling the volume on your Hisense 4K UHD TV, you can contact Apple Support for help.

I hope this helps!

See: If the volume buttons on your Apple TV remote stop working - Apple Support

The Siri Remote send IR volume control signals directly to the TV. CEC is not involved in that.