5G Development Kit blocked on T-Mobile network?

Trying to get Rasp 5 with Sixfab 5G Development Kit to work on T-Mobile in California. The modem worked, and a download speed test showed 15MB/s down and 20MB/s up. So far, so good. Ran the same test again, and suddenly it was 0.7Mb/s down and 1.04Mb/s up. Then it finally quit.
atcom AT+QWNINFO says “No Service”.
Is t-mobile throttling me or locking me out?