Power off the device by unplugging from power outlet.
Remove the existing SIM card found on the right hand side of the modem by taking a paperclip and push in to release the SIM card and remove it.
Insert the SIM card on the right hand side of the device with the pins face down and the wedge toward the modem until you feel it click into place.
Power your modem up and give it about 5 minutes to power up. This should restore your internet service.