Hello admin@hiecon.com,
You can totally do so! Go to System Management --> Dual WAN ---->Interface Setting Table.
Once you're there please select either WAN 1 or WAN 2 and then click edit and scroll down to Protocol Binding Table, click add, select service as All Traffic (or an specific service of your choice), source IP to begin and end on the server's IP address, Destination IP as 0.0.0.0 to 0.0.0.0, Interface as the WAN interface you'd like to use and click on the checkbox under Status then save your configuration and you're done!
Have a great day!