This FAQ discusses possible error messages that begin with Unable to Establish A Connection

The most likely reason for this type of message is that your security system is busy with a Panel Sync.

• Loss of your security panel's primary and backup connections to Remote Services at once (cellular network and Internet, respectively).
• The cellular hardware in most systems is not user-serviceable. Consult the documentation that accompanied your control panel.
• You may be able to partially address the problem by rebooting your Wi-Fi router, which the security system depends on for access to the Internet. 

• Loss of connection between your panel and the sensors on your premises (this is unlikely to occur with all sensors at once).