Do like this:
Create a Template Command to be sent to the device. More info on Template Commands here:
Note that you can send the command over SMS or GPRS (tcp or udp). Or if the device supports GPRS commands, you can setup the command to be sent once the device connects / is connected.
The Terminal is a very useful tool when testing commands.
Create an Event Rule with an "Geofence Expression" in Step 4. More info on geofence event rules in this video guide:
Add a "Command Notifier" in Step 5. set it to send "back to device". Select your Template Command. You can set the command to be sent when the event starts or ends.
Now save and test!
Do you want to send an SMS to reset the device if it is off line for a longer period of time? Use an "Offline Expression" in Step 4. of the Event Rule Wizard, and create a Template Command to be sent over SMS to be used in Step 5. Command Notifier.