MAX Transit Can't Use 2 SIMs

I’ve been using a MAX Transit LTEA for a couple years now. I’ve never been able to put 2 SIM cards in it. If I do, it can’t get online or ever connect. I use one at a time and they both work fine. What should I do? Thanks!

Max Transit = 1 modem, 2 sim slots
Max Transit Duo = 2 modems, 4 sim slots

What is the model part-name/number exactly? How many sim slots are there total?

Single modem devices have 2 sim slots for failover/redundancy/bandwidth-cap-reasons - actively utilizing only 1 sim at a time.