Sounds like you have a Max Transit Cat-18 device.
This model for some reason does not work with T-Mobile. I tried 4 different T-Mobile post-paid SIM cards in the last month on the device, and none of them worked. These cards were pulled from working devices and then reinserted into other devices and all worked without issue except for the Cat-18 model.
There are a bunch of threads in the forum with the same issue (https://forum.peplink.com/t/max-transit-cat-18-not-working-on-t-mobile ), and I do not believe it has been resolved at all yet.