Follow these steps to install SIMs:
Place the router bottom-side up on a suitable work surface.
Press the SIM Tray Ejection Tool into the tray ejection hole for SIM1 and remove the tray.
Insert an activated 4FF-sized SIM card into the SIM1 tray. The SIM only fits into the tray in one orientation.
Slide the tray and SIM card into the SIM1 bay on the router until it clicks into place.
(Optional) Insert a 4FF-sized SIM card into SIM2 bay using the same method.