We have received feedback that some Jolla C2s have had problems with SIM card. For few cases fastboot flashing fixed that. Why this occurs in the first place, we do not know yet.
Please find fastboot flashing instructions from here:
If you see that there are issues with SIM card not recognized or mobile data not connecting please help us out and try instruct reflashing before instructing sending devices back to us for repair. Surely, it is possible that a device has a hardware defect and those should be returned back.
Raine from Jolla