Subscribed via Apple TV and can’t log in via Apple TV.

There are multiple types of subscriptions that are not exchangeable. They have different terms and conditions, different perks and limitations.

• Subscriptions as started in the Apple TV app, are the in-app Channel subscriptions, and need to be watched in the Apple TV app. The subscription for this will be automatically recognized from the active Apple ID account.
• Subscriptions as started in the content provider app on an Apple device, will need to be watched in that content provider app on an Apple device. For select apps, content can be linked from the Apple TV app (Up Next, etc.), as ‘connected app’, but the separate content provider app will still be needed. The subscription for this will be available from ‘restore purchase’ from the active Apple ID account.
• (Another option is through an account with AMC+ directly. Then use the separate content provider app.)

Both types ①+② will be billed through Apple, and will show up in your subscriptions overview. The Channel subscription will have “tv channels” behind its name.