Hello,
Thank you for your question.
We would suggest to use sleep modes, The full description of sleep modes can be found in our wiki:
https://wiki.teltonika-gps.com/view/FMB920_Sleep_modes
Depending on the modes, GSM module can be offline: Deep sleep/Ultra deep sleep, so the device will not respond to SMS/GPRS commands. GSM module stays online in: GPS sleep mode/Online Deep sleep mode, device will respond to SMS/GPRS commands. However, the devices will not leave the sleep modes until mentioned conditions are met.
There are no specific command to force device to leave sleep mode. In your case you could try using SMS/GPRS command for "cpureset". The device would leave the sleep mode after reset, but will go back to sleep mode if all the conditions to enter the mode reached.
Hope this helps.
Best Regards,