The Load Balance setting is a little confusing.
If you are running Dual WAN then enable Load Balancing regardless of any other settings.
The settings in the Link Backup tab will actually determine how the WAN links are used.
In your case:
your Primary WAN interface would be: WAN
your Backup WAN interface would be: USB Modem
The MODE would be: Backup when all primary WANs fail
NOTE: In my case, I use both WAN and WAN/LAN as Primary since I have 3 WAN connections.
ok, then I did the correct configuration about the failover.
About NAT-DMZ, should I include all the inferfaces in the rule or just the primary wan?