According the instructions for the Coban GPS303-G tracker if when no WCDMA/GSM reception, green led ON.
GPS trackers rely on satellites to provide location data. If there is no GPS signal in the area, the tracker will not be able to provide accurate location data. GPS signals may be obstructed by tall buildings, heavy foliage, or other natural or artificial obstructions. There may also be a problem with where in the car and how the tracker is located. For example, it is located in a shielded metal box, etc.
So please, check the location of the tracker and try moving with tracker to an open area with a clear view of the sky.
Regarding commands for the tracker (including restart), it is best for you to contact the tracker manufacturer directly. Also, such commands are often described in the instructions for the tracker https://content.gps-trace.com/assets/a901d022-d937-4543-afef-e9a1a4dc4c7e.
If you have further questions, please let us know.