Hello,
It is unfortunate to hear about your incident.
Unfortunately, there is not much Teltonika can do in this case. Any restrictions within device can be easily removed by performing factory settings reset and they need to be set by connecting to the device.
Router modem's IMEI can be used by network provider to block the device from accessing the network. The IMEI should be printed on the packaging the router came in.
Keeping your device connected to RMS might provide you its approximate location once the device comes online with mobile connectivity, but GPS monitoring must be enabled in RMS. To do so, select the device, in the Devices section, click on Management tab, select Set Monitoring Update Period. Enable GPS monitoring and set update period frequency.
Regarding SIM serial number, I assume you refer to ICCID number. The only way to get it remotely is to connect to the router via WebUI Status -> Network -> Mobile section, or SSH and enter this command: gsmctl -A "AT+QCCID", however, the device needs to be online and connected to RMS.
Best regards,
Žygimantas