Unable to control TV volume with Apple TV remote

I just got a new TV, and after I connected my Apple TV I can't control the volume using the remote. I have reset the remote, the aTV, and the TV (repeatedly). I have HDMI CEC turned on in the TV settings. But in the aTV settings, the only option for volume control is IR - which doesn't seem to be working. Is there a way I can manually pair the IR remote to the TV controls? Or a way to manually force it to use CEC instead? Some other reason why this wouldn't be working?

The remote does successfully control power of the TV, and it works for everything else. I can also successfully pair the TV's own remote to control the aTV. It's specifically ONLY getting the aTV remote to control volume that doesn't work.

The TV is a 'Cello', which is a budget brand.