You can configure your Firebox to send traffic through a modem when the Firebox cannot send traffic with any external interface. For a list of supported modems and Fireboxes, go to About Modem Interfaces.
The modem is available as an external interface, and modem failover is enabled. The modem has a higher metric (lower priority) than other external interfaces. If all other external interfaces become unavailable, traffic automatically fails over to the modem interface. To add a modem to your Firebox and configure the settings, go to Configure a Modem Interface.
You can also configure branch office VPN connections to fail over to a modem interface. For more information, go to Configure VPN Modem Failover.
To add a modem interface as a multi-WAN participant, go to Configure the Failover Multi-WAN Method.