A common request is to have all network traffic exit and return on a singular WAN interface (primary), but still have other interface(s) take over during a failure or outage, then have traffic return to the primary interface. This configuration requires WAN Balancer as well as WAN Failover.
WAN Balancer must be configured to force all traffic out of the specified primary interface. Two specific configurations are required: A rule or rules to force any traffic coming from the internal interfaces to exit the primary external WAN interface (Apps > WAN Balancer > Route Rules), and to have the Traffic Allocation weights set to 100 on the primary, and 0 on all others (Apps > WAN Balancer > Traffic Allocation).
NOTE: In the example below, I am showing that the value of the Source Interface condition is set to Any Non-WAN to specify any internal interfaces to be forced to the primary interface.
WAN Failover then must be configured to have tests per each interface, to test for any failures on any interface (Apps > WAN Failover > Tests). This can be viewed more in-depth under https://wiki.untangle.com/index.php/WAN_Failover. WAN Failover takes precedence over WAN Balancer in that when an interface failure occurs, the WAN Balancer rules that were previously configured no longer apply and all traffic will use the next listed interface indicated by interface ID (below shows  for external, and  for Failover.