@Charles_Ash, Are you sure the devices are actually connecting to the T-Mobile network?
I tried 4 different T-mobile postpaid SIM cards at the end of August 2021, with a Transit Cat-18, and none would actually ever connect to the network.
I don’t have a Cat-18 on hand to test with at the moment, otherwise I would be happy to find out if T-Mobile was going to work with the device finally. I have been forced to swap out some SIM cards to other carriers due to this.
First question - Is the cellular light flashing on the device? Ignore the WIFI light if it might be flashing. Peplink | Pepwave - Forum
Secondary link to original thread bringing up Cat-18/T-Mobile issue - Max Transit Cat-18 not working on T-Mobile