If setting up cellular connectivity, do the following:
Turn the router over and remove the screw to the SIM door.
Lift the SIM 1 card slot and insert an activated 2FF-sized SIM card. Insert the card with the metal contacts facing the interior of the device when closed and notch end up. Refer to the image above.
Insert a second activated SIM card into the SIM 2 card slot, if desired.
Flip the SIM slot(s) down then close and secure the SIM door with the screw.