Hello,
I have tried to perform SIM switch both ways via WebUI and SMS commands, but did not experience any issues.
If you have performed firmware update with Keep settings option set to on, could you reset device to factory defaults (in the WebUI go to System -> Backup -> Restore default settings) and check, if the issue persists? Please note that this will delete your current configuration.
If device reset does not help, could you describe, why does the device need switching off?
Could you provide a troubleshoot file to check the SIM switch sequence? First, replicate the issue by issuing SIM switch, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there. The logs in the file may provide more details into possible causes of the issue. Attach the file by editing your question.
Best regards,