Yeah all four US carriers are a challenge. Ping one of our disti’s/sales guys as our SIM cards might be able to roam across all of the operators. Verizon is the difficult one. I think you might have to have a roaming SIM to do all of the operators and then a specific Verizon one.
One thing I would say about the modem you have just pointed out is that it supports band 17 where AT&T use band 12, band 12 is a superset of 17 i.e. it includes it but is bigger so you might be miss some coverage and throughput capabilities.
Why do you want to be able to use all of the carriers? Is it coverage? If so, as I understand it, Verizon is difficult to get away from.
Regards
Matt