When did you get this iPhone? When did you activate it (first use)?
The offer has to be redeemed within the first three months.
For eligible devices, the duration of the offer changed for devices activated after July 1st, 2021.
All offers transition into a paid subscription after the (extended) trial period, unless canceled before the end. Cancelation during the trial period expires the subscription immediately, and the trial can’t be reactivated then.
The offer has to be redeemed on the new eligible device. Once you have the subscription, it would be available on any of your devices.
Read here how to redeem: Apple TV+ offer - Apple
Read here about the conditions: Apple TV+ offer - Terms and conditions - Apple
Read here if you have trouble redeeming: If you can't redeem your promotional trial of Apple TV+ - Apple Support