Hello, sutin-boris ,
First of all, try to check the next things:
1) Sim-card balance and traffic limits.
2) Additionally recheck device configuration: data sending interval and APN configuration.
You can also check the Toolbox of the unit. There you can see if your device is trying to connect or probably there is a problem with incorrect data in messages.
By the way, if this info does not help you, you can write us to email@example.com indicating login, unit ID and device type, we will try to help you.