Apple TV 4K + LG Computer Monitor: Apple remote has but loses volume control

**Equipment:** * Apple TV 4K with remote. * 43" inch computer monitor LG 43UD79-B 4K with built-in speakers, has its own remote. **Connection:** HDMI, from Apple TV to monitor **Problem** When I first connected the Apple TV to this monitor I was able to control the volume on the monitor using the Apple Remote. Handy, one remote for everything! After a couple of days and for no discernible reason that functionality disappeared. What did work was the monitor's remote for the volume. But that means using two remotes. I fiddled with the "Volume Control" settings in tvOS to no avail. I then power-cycled the Apple TV, fiddled with the settings again and the functionality re-appeared. Then a while later, the functionality disappeared again. I'd rather not do this whole powercycle + fiddle with settings dance every other day. Does anybody have an idea what could be causing this and how to fix it?