This other modem is a USB stick, I use it only for checking that the SIM card is not broken.
I’m pretty sure the SIM card is inserted properly in the MIFI - I tried reinserting it several times and it didn’t work even once. I never had issues with SIM detection until the modem started misbehaving. I wonder if there could be some problem in the connection between SIM card and modem - since the SIM slot is on the MIFI board and not on the modem. Is there any way to check that?