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Try restarting your TV and checking for software updates. If the issue persists, log out from the app on your iPad and TV, then log back in on both devices. You can also reset your password or reinstall the app on your TV. If none of these work, contact the service’s support for further assistance.

Is this about an Apple TV+ subscription, or a different streaming service?

• For signing in to the Apple TV app on third party devices, do so through the in-app Settings, using the Apple ID account to which this Apple TV+ subscription is linked.
• When signed in to the Apple TV box, you would be signed in automatically for Apple services through that Apple ID account. Note that the Apple TV box allows for multiple users, and the wanted account should be made active first.
• For third party streaming services with payment through Apple, some apps on the Apple TV box require to “Restore Purchases” instead of signing in, e.g. when originally started on a different device. This subscription type through Apple for these third party apps wouldn’t work on third party devices.