Can I swap SIM cards between NightHawk MR1100 and LTE Modem LB11230

In general, swapping SIMs to a comparable type of device should work. I've heard of folks having success moving a SIM from a Mobley to other hotspots like a Unite Explore or Netgear Nighthawk M1. But AT&T has certainly done things to try to prevent that, such as SIM locking on Mobleys or tying a device's IMEI to your account and only allowing that. You'd either have to do research to find someone else who has done what you're trying to do (recently) or just give it a try yourself.