T-Mobile USA review: Line cancellation

Approximately 4 years ago I purchased a 2 galaxy watches with T-Mobile. one of the watches was damaged and T-Mobile exchanged it for another watch and that required opening another account. The store clerk assured me that the first account would be canceled., After receiving my first bill I noticed I was now being charged for the 3 watches they are under wearables in the T-Mobile bill statement. I called again to explained that I only had 2 watches and the extra line they charged for was no longer owned by me. They said they would refund me and cancel. I trusted T-Mobile with that. So today October 31, 2023 I decided to check my bill and noticed T-Mobile was still charging me for that 3rd line (wearable). I reached out to customer service and they said they have canceled it and that they would only refund me $30. I brought up the fact that they owe me about 4 years worth, so they flipped it on me saying I should have checked my bill monthly and called them and they cant go back that far. Why after multiple attempts to cancel and being told it was taking care of I should have to keep calling about the same issue. I trusted the problem to be solved. I been a customer for about 20 years. I'm so disappointed in T-Mobile. I added a line about a a few months ago and was told I get a free phone and then they said no free phone because of my plan. T-Mobile you treating your faithful customers that been with you for over 10 years like crap. I should be refunded for the watch.