Billing issue t mobile wont fix

How do I get T-Mobile to honor the price that they gave me when I signed Up? I have the price in writing that they were supposed to charge me every month and I have contacted them at least 10 times and they keep charging me the wrong amount or the representative acts like they’re going to help me and escalate my claims and I never hear back from anyone. Now they’re saying that I am past due and they’re charging me a different amount because I removed auto pay. Not sure how to proceed with this. I can’t afford what they’re trying to charge me.