Using your TV app to watch a movie

Xfinity box

Thank you for that clarification. Unfortunately, the Apple TV+ app on that device is limited to Apple TV+ subscription content. The difference between the Apple TV+ app and the full Apple TV app (with Apple TV+, purchased content, and more channels) is explained in more detail here: Get the Apple TV app or Apple TV+ app on your smart TV or streaming device - Apple Support.

I think I can guess why Xfinity wouldn’t want to directly support the full app with additional (competing) services on their own device.

I’m afraid you’ll need a different device or different connection to see your movie on TV.

One which device did you intend to watch this?

List of Apple TV app-supported devices - Apple

The purchased movies on your iPhone (from Apple’s iTunes Store) are automatically synced to all your online devices with the Apple TV app, when signed in with the same Apple ID account. See the Library tab of the Apple TV app.

Get the Apple TV app or Apple TV+ app on your smart TV or streaming device - Apple Support

All the ways to watch in the Apple TV app - Apple Support