peplink MAX HD2 Dome Quick Start Manual page 8

If you repeat the steps from 4b) on Cellular 2, both cellular WANs will use the SIM cards
from the SIM Injector.
c. If you want to select a specific SIM, in the Cellular Settings, type ":" and then the number
of the SIM slot. For example, from the following screenshot, you will choose the SIM
cards on slot 4 of the SIM Injector. Use multiple lines to specify multiple SIM slots.