Unlocking my device - Cell Phone

I purchased an iPhone through T-Mobile, and I have completed all the payments for the device. There is no remaining balance on my account. I have already canceled my phone number because I switched to a different carrier. However, the phone is still locked, and I am unable to use it. The cell phone has been in use for the last 6 months. It is part of a single-family plan, and there is no one else on my plan.

I would like to note that the device was sold by T-Mobile and has not been reported as lost, stolen, or blocked. Additionally, my account is in good standing with T-Mobile. Despite these circumstances, when I visited the T-Mobile store for assistance, they informed me that they couldn't provide a solution and advised me to contact Customer Support over the phone. However, when I reached out to Customer Support, they mentioned that they are unable to access my account since my number has already been canceled and directed me back to the store.

I have been going back and forth between the store and Customer Support without any success in resolving this issue. I sincerely appreciate your help in resolving this matter.