T-mobile charging me after cancellation

My family, and I decided to go back to T-Mobile during an upcoming deployment back in 2019. My duty station changed, and my family and I moved into a new home. We did not have cell reception with T-Mobile and transfer our three phone lines, and requested to cancel our sons account. We went through loops to have my husbands line canceled, and was charged again for our sons account after the cancellation request. I was assured by a T-Mobile rep this would be fixed. I was refunded 74 for my sons tablet. Two months go by and I receive a charge of 144. I call T-Mobile, and they see the refund request was put in, and cancellation request was noted and not processed. The rep then cancels the account, and assure the balance would be cleared. Long story short…. 8 month later this account after monthly calls, and constant assurance of a resolution…. A debt appears on my credit report. T-Mobile reps still trying to figure out why this issue has not been resolved. I was informed by a rep that I should pay the amount for a quick resolution. They can clearly see a they are in error, but all seem clueless on how to fix this. I have waited for 30 min to an hour for a rep to get on the line for the call to drop shortly after. This issue is still unresolved, and what a headache dealing with the staff who are poorly trained. This has been a nightmare.