Credit/Refund for services not used

I purchased a phone at the apple store in Green Hills(TN) in November 2024 intending to finance it through T Mobile. I wasn't charged for the device and the service in December, so I called the apple store (Green Hills) and was informed that a billing cycle will be skipped. However, I still was not charged from January -March. 

Concerned, I called T-Mobile in early March and was told that my account has a past due balance and will go into collections. An agent processed a credit for December for the service and late fee since he saw that I didn't use the service.  I went to the nearest corporate T Mobile store and was told I would need to pay for the device in full in order for it to not go into collections.

I paid for the device in full on March 4, 2025 and was told to call customer service to get credited for the unused services for Jan.-Mar.  I didn't use the services (POSTPAID account) so I was informed at the store in Nolensville Pike (Nashville TN) that I should be credited for the January-March since she saw that I was credited for December's unused services. I called customer services again and explained everything and agent told me that I will be credited for all three months since he was able to see that i didn't use the services. He said I should wait a couples weeks to get an email confirmation for the credit to my account. Well I never received that email. 

I called on 4/22/25 to check on said postpaid account. Ofc I was not shocked, since this has been an unnecessary headache, when the agent told me that there is a balance plus late fees on my account.  Bare in mind that this account was canceled and shouldn't have an balance. Agent said that I needed to  call the Apple store that I bought the device, and when I called they couldn't access my account so I was sent right back to square 1 which is to contact T-Mobile. 

This has become my last resort. I cancelled my Postpaid account by paying off the device in full, yet I am still being told that I need to pay a balance and late fees for services that I did not use and was told would've been credited to my account. I am seeking a resolution and a proper credit for the months I was incorrectly charged and now told to pay for.