Mod Edit (@harrisonhjones): Solution: T-mobile managed to remove the data allowance from the plan while activating the SIM. Swapped cards with my phone and it hooked right up, no problem.
Activated a new third party SIM card on a previously claimed device, got to the point where it attempts to connect to the particle cloud and gets stuck.
I’ve tried fixing the keys per other posts, but no dice. Unclaimed the device in case it was getting confused, didn’t help.
Don’t know what else to do.