T-Mobile Experts and Management have lied to me and I do not know what to do next.

I have been contacting T-Mobile since December 2021 about a promotion that is not being honored and T-Mobile acknowledged fault on their part and they stated it will be fixed on next billing. It's never fixed and I was told by Ash that I would get a follow-up call on 1/17 never did, Steven said he would call me back in 24/48 hours on 4/5 never happened and now I am being told to hold on and no one ever comes back and I stay hanging. Any advise on how to proceed?