RAK8213 device busy

I can’t access the card, I always get “Device or resource busy”

The card is installed on a RAK7391 gateway board, it is on the miniPCIe slot #2.

I can see the device mounted in /dev as ttyUSB3 but I can’t get it via a serial communication. When I don’t insert a SIM card, I can access the device using minicom and I got responses for AT commands (not all ofc). But when I insert the card, the LED blinks differently (When checking the datasheet, it is 1800 ms High, 200 ms low, so it’s idle) and I can’t access the device. Whether via a Python script or minicom, the result is always Device or resource busy: '/dev/ttyUSB3'

Thank you in advance
Best regards