Can't Cancel Internet

Back in May (now August 2024) I dropped off my home internet equipment at my authorized store. They told me that I needed to call the provided number to cancel my service. I have tried many times to do this to no avail. I do not have my 6-digit pin. I know my username and password to get a new pin, but I can't get into my account without providing my T Mobile phone number, which I never had. I am getting charged monthly for a service in which I am not using and returned all equipment for. 

Calling customer services gets me nowhere. I live 94 miles from the nearest store and that is their only solution for me. I don't understand how they can continue to charge me for an account I do not even have access to.