Why is my service throttled to 2 mbps?

Since I was forced from my old unlimited plan to the new one ($5 increase), I’ve gotten texts for two months. The first one is a warning that I’m approaching a 2gb cap and the second tells me I’ve hit it. My service throttles to 2mbps on an iPhone 16e that has five bars of 5g. T-Mobile couldn’t fix it on their end. What is going on?