The offer for an extended free trial to Apple TV+ is available to the first registered user of the new, eligible Apple device, unless the user has previously had access to Apple TV+ (“already tried it”).
The offer for an extended free trial to Apple TV+ should be redeemed on the new, eligible Apple device; not on a different device. Once redeemed, the subscription service is accessible on any of your devices when signed in with that same Apple ID account.
The offer for an extended free trial to Apple TV+ should be redeemed within the first 90 days. Trying to redeem after that time, would result in seeing just the regular 7 day trial.
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 promotion for an Apple subscription - Apple Support
↳ Step 5 says to contact Apple if the prior steps didn’t bring a solution. They can assist.