Hello, StavrosZampetoglou ,
Please write us to email@example.com specifying login and unit ID, then we can check your issue additionally.
By the way in such cases, first of all, check on the tracker side:
1) SIM card balance and traffic limits on it
2) Data sending intervals and tracker operation mode
The tracker may have entered sleep mode or some settings have gone wrong.
In most of such case there is problem with Sim card balance or tracker configuration.