Promotional Trade-In Credit Never Applied to Account?

T Mobile is a disastrous. I have been a customer of 6 years and recently in Jan 2024, i added two lines over the phone with customer service with the agent told me to give me 2 iphone 14 promax devices with a trade in of 1 iphone 12 pro and i phone promax 12 with a 800 dollar discount on each device price. I shipped two devices of mines to them through a  UPS prepaid envelope ( supplied by t mobile) in May or April after my several follow up for the UPS mailer envelopes. Since then they never applied discount and I have been paying for each device full cost per month (45.84 per month per device total 90) and i paid almost 10 months in between several times, i called customer service and they said they had  a problem. I stopped payments and they  are cutting services even after agreeing that the problem is from their end and one customer service agent  had done wrong. Ultimately, customer is suffering, i have been in calls with agent each time a minimum of 1.30h, No resolution. T mobile is cheating customers. I hope this will bring to their leadership team. i will quit T mobile