I have Free Apple TV from T-Mobile. How do I sign in via my Apple TV device (2nd gen)?

T-Mobile’s offer still needs to be redeemed, as T-Mobile describes, before the Apple TV+ subscriptions gets linked to your Apple ID account. Then once signed up, you should have access to the Apple TV+ content on any of your devices when signed in to the device or app with that matching Apple ID account.

Apple TV+ on Us: Subscription Included With Your Plan | T-Mobile

↳ Activate your Apple TV+ subscription

↳ Got questions? (FAQ)

↳ How do I redeem my Apple TV+ On Us?

• The Apple TV 4K (2nd generation) [A2169] applies the Apple TV+ subscription when the matching Apple ID account is active for this device. Find Apple TV+ content inside the Apple TV app.
• The legacy Apple TV (2nd generation) [A1378] doesn’t support the Apple TV+ streaming service.

All the ways to watch Apple TV+ and more - Apple Support