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.

Wait a few minutes and try again.
If the problem didn't result from a sync operation in progress, it could be happening for other reasons, including:
  • 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).
If the problem persists, contact your security professional for help.