Certain Cradlepoint products, such the MC400-5GB modem, have a removable tray for the SIM cards.
Insert the SIM tray key into the small hole to remove the SIM tray from the modem.
Place at least one SIM into the tray - if only using one SIM card it must be in SIM 1 position. SIMs only fit into the tray one way.
Slide the SIM tray back into the modem until it clicks and is flush with the surface of the modem.