There are two functions provided for users - Smart Link Backup and Load Balance. If users select DMZ in setup page, users could not do the Dual WAN setting here.   

If Smart Linkup is selected, you only need to choose which WAN port is the primary and then the other will be the backup.

Network Service Detection
This tool can detect the network connection status of ISP by pinging Default Gateway, ISP Host, Remote Host or DNS Lookup Host.
Retry count: The count of ping. The default is 5.
Retry timeout: The interval between two ping actions. The default is 30 seconds.
When Fail:
Generate the Error Condition in the System Log: The Router will generate the System Log when ping fail to inform users that the ISP connection is disconnected.
Remove the Connection: This WAN Interface will be suspended when the network connection to ISP is not active. The traffic on this WAN will be dispatched to the other WAN port. Once ISP returns to connect, the traffic will be dispatched back.

If you check this Detection box, you have to choose at least one option from the following four items.
Default Gateway: If you check this item, the Router will ping the default gateway first.
ISP Host: After ping Default Gateway, the Router will ping ISP Host “Retry timeout" later. The ISP Host is provided by ISP.
Remote Host: Enter the IP address of Remote Host that you’re going to ping.
DNS Lookup Host: Enter the Host Name or Domain Name that you’re going to ping.

Click the Save Settings button to save the Dual WAN Smart Link Backup settings or click the Cancel Changes button to undo the changes.