@Cher Hi, looking at your screenshot, I think there is a bug in the modem firmware. If you look at the logs on the right you can see:+CPIN: RE DY
This means the sim card is detected and ready. However, one letter is missing. This is supposed to be +CPIN READY. So I think the serial tool is therefore failing to interpet the correct string.
Can you query the CCID/IMSI? If it returns a number then the sim card is for sure working. You can also try updating the firmware