Escalate complaint over trade in promotion where no monthly bill installment was promised

Hi - In March I was told by T-Mobile representative that I could upgrade my 2 lines of iPhone 12 to iPhone 15 for about $100 each for the device and I would only pay about $10 monthly equipment installment for each.  Now it's June.  I received my bill an it's $63 higher for 2 lines.  I called. The representative actually read the transcript and confirmed that what I just wrote was correct.  And T-Mobile rep misspoke.  They meant to say that I would get about a $7 discount from normal equipment fee.  All they said that they were sorry.  And they did mess up.  And that the previous employee would be disciplined.  And I could return my iPhone 15's and go back to the iPhone 12's.  Talk about hassle factor!  But that I would still pay more per month because there was no promotion available.  In other words, there was no way forward without paying more… even if I somehow went backwards to iPhone 12's.  I've been a customer for 17 years! anyone have any suggestions on how to complain outside of T-Mobile?