When enabling cell fallback option be aware that a DNS that is reachable must be defined in the event of a fallback. This will allow emails for SNMP traps and any other alerts to continue to be generated
Example if a local DNS is defined and that unit loses the local network it will fallback to the cell interface. When that happens the local DNS becomes unreachable. A second DNS should be defined that is reachable by the cell interface. For example I supplied the following google DNS server to the device: 8.8.8.8