The connection between the watch and the app depends on the data on the SIM card. If the SIM data is depleted or the data package is not activated, it will not be possible to send location information and will be reported as offline in the app. The SIM card can be activated by inserting it into the phone and calling another phone number + loading any web page (this will also activate the data pack, WARNING: the phone must have Wi-Fi turned off for mobile data to be used). 

It may also be that there is sufficient credit on the SIM to make the call but not to renew the data package. 

Restoring the data pack / topping up the SIM credit should resolve this issue.