I was under the impression that Dolby Vision was only available on HDMI inputs 3 and 4, but I’ve given it a shot on input 2 (input 1 is taken by my Nintendo Switch) and it looks like it works even though “Enhanced (Dolby Vision)” isn’t an option under External Inputs in the X90K’s settings.

In any case, I’ve gone out to buy another certified 48 Gbps HDMI cable and that appears to have solved the issue. Lesson learned: not all cables are created equal!

On your Apple TV go to Settings > Remotes and Devices > Home Theater Control > Set “Control TVs and Receivers” to ON.

Double check whether HDMI-CEC is enabled on your tv if the above doesn’t work.