did you try AT+CSIMSEL=1 ?
see 11.4 AT+CSIMSEL Switch between two SIM card in this document:
The command is used to select external or embedded SIM card. NOTE
Embedded SIM card supported by customization. Customer should provide informationwritten into USIM chipset.
The command is disabled if the embedded SIM card isn’t exist, i.e. standard hardwareversion.
1 – external SIM card
2 – embedded SIM card