I come across the same situation here, I would like to tell if another, un-activated slot has a SIM in it or not. If I can do this, then I can workaround a situation that using UIMS AT command to change to an empty slot will make the modem unable to use this command to change back to the slot with a SIM.
You can try using AT!UIMS command to switch to other SIM with some AT commands as below.
Please make sure the APN and frequency band are corrected with each SIM card to connect cellular network.
I have attached the log file for your reference.UIMS.txt (4.9 KB)