I have been having this issue since Christmas. I got my daughter a Series 5 Cellular watch, and since she doesn't have a phone, we set it up as a family watch on my phone. It's her apple ID and everything, just set up on my phone. I also have my own watch but it is an older one. Series 3 I think. I recently switched over to visible to support my daughter's watch. We wanted to set up her watch with my visible cellular but every single time I go into the watch app and press the cellular watch setting in her watch settings, it loads for a bit and the button, "Set up cellular" is grayed out, and below it says "the visible account on this iPhone cannot be used to add a cellular account to "(my daughter's name)'s Apple watch".
See the attached photo for the screenshot
