Apple TV Remote can't control TV volume any more (auto is "off")

One of my two AppleTVs just started acting strange when I use it to change the TV's volume: when I touch the +/- volume buttons it complains that I have not set up the volume control & asks whether I want to do it now.

If I choose Yes, I see that the Auto volume setting is "Auto (off)" . I presume that I want Auto to be "on" but I can't doit. I can uncheck and check Auto but nothing changes.

The AppleTV and remote used to work fine with the TV. All that changed here is that I moved the AppleTV to another room where it worked with another TV for a few months. Both TVs are LG, both are approx 5 years old without "CEC" settings.

I have restarted the TV, and I just reset it to factory settings; no change.

I can't figure out what changed or how to get the remote working again. It's a pain in the neck; anytime anyone's finger touches a +/- key the message pops up.

Any tips?