Meraki MX devices do not have automatic security mechanisms to revert to the previous configuration and restore the connection if both WAN interfaces are intentionally shut down.
For good practice, it is best to have a pair of MXs in HA.
If an intentional shutdown is performed manually, there is not much you can do other than access the MX locally and reactivate the interfaces.
To prevent this from happening, only those who really need access can do so.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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