I happen to have a newly bought T-Mobile prepaid SIM card. It offers limited "high speed data" (4G, 5G, etc) and unlimited "slow speed data" (2G, 3G). Once I ran out of "high speed data", my service was supposed to fallback to 2G, but it doesn't work. I asked why via customer support and the consultant said that they know that it doesn't work, even though they're selling it. Took me 60 minutes on the phone to get to that point. I just gave up. I ended up paying 30 dollars for around 3gb of data and wasting a bunch of time. T-Mobile has been horrible in my limited experience.