Hi there Calbra4x4-,
If we understand correctly, you use Family Sharing with an Apple TV+ subscription with another Apple ID, but you're prompted to sign up when you try watching content on a different device. Is that right?
First, we want to confirm. Are you only having trouble accessing content from the Apple TV+ subscription? Knowing if only this function is affected will be helpful. Some things you could check are iCloud storage sharing, Apple Music, or general purchasing sharing. Let us know if any of those work for you.
Depending on what you find, you may want to make sure you're signed in under Settings > [name] > Media & Purchases with the Apple ID that you're using for Family Sharing.
If you're having issues with other aspects of Family Sharing, this might help as a start: Set up Family Sharing in the Apple TV app - Apple Support
"You can’t activate Family Sharing directly in the Apple TV app. You must set up Family Sharing on a Mac, iOS, or iPadOS device.
- To activate Family Sharing in the Apple TV app
, follow the steps in the Apple Support article Set up Family Sharing."
Kind regards.