What are the differences between SIM A and SIM B on both modems?

SIM A and SIM B slots are the same, except by default setting the preferred SIM Card is SIM A. If both sim slots have sim cards in them, SIM A will be selected as default.

This setting can be changed via device Web GUI, Network -> Connection Setup -> Click Edit at WAN-1 -> Preferred SIM Card in 3G/4G WAN Type Configuration.

If one SIM card is inserted, the device will automatically detect the SIM card, no matter if it is in SIM A or SIM B slot.

SIM B can also be enabled as the fail-over to SIM A under the setting - Preferred SIM Card in 3G/4G WAN Type Configuration menu.