Experience Signature plan throttled for two months and nobody at T-Mobile can help

Since the forced transition from ONE to Experience Signature, my phone has been throttled to 2.5 mbps after 2 GB of high-speed data for two months now. Nobody at T-Mobile seems willing or able to fix the problem. I am in a major U.S. metro area with five bars of 5G UC and have a fully updated iPhone 16e. What do I need to tell T-Mobile to do?