Please make sure that your SIM card has a working Data Plan. The watch will not be able to connect to the server without mobile data.
To know if your watch is online, you can check on the home screen and look for the "Connection Icon" as shown below
If the "Connection Icon" is missing, please read below for possible reasons and solutions
Due to inactivity, it is possible the watch has disconnected from the server to save battery. The watch will reconnect automatically on its own but you can force the watch to reconnect by restarting it. (How?)
A weak or no GSM signal in the area can temporarily disconnect the watch from the server. The watch will reconnect as soon as it receives a stable network signal
Your Data Plan Package might have expired. You can use Post-Paid SIM cards so you won't have to worry about expiring SIM card loads. (SIM card guide)