I have put my SIM card in a GPS tracker. What do I need to do to get the coordinates of its location sent to me by text message?

To receive the coordinates, you need to follow the guided procedure described in the device’s user manual. If you are using a GPS tracker with SMS-based commands, send a text message with the command to request the location to the number of the Things Mobile SIM card. 

Please note: In particular, remember to put 00 or + before the Things Mobile number (for example 00882... or +882...) when you send an SMS message from another operator to a Things Mobile SIM card. Similarly, remember to put 00 or + and the country code before the number (for example, 0039 or +39 for Italy) when you send an SMS message from Things Mobile to another operator.