Hello,
Thank you for your question,
FMB devices enter/leave the sleep mode when the specific conditions are met:
The full description of sleep modes can be found in our wiki:
https://wiki.teltonika.lt/view/FMB120_Sleep_modes
There are no specific command to force device to leave sleep mode.
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. In your case you could try using 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,
Regards.