I had a similar problem - AppleTV+ only worked on one of my 3 Contour boxes. I'm new to Cox, and I only realized later that my Apple subscription was showing "free trial" for TV+. My previous ISP included TV+ in their bundle, and when I dropped them they continued my coverage briefly. When old coverage expired Apple came through with the free trial - but only on one device.
This is likely different from most of postings for this problem, but maybe somebody else is in the same boat (not that free trial is a bad boat to be in).
