Hello phoutsavong,
To learn more about load balancing and how to configure the WAN Interface on the RV320, you can refer to the article, Dual WAN Configuration on RV320 and RV325 VPN Router Series. This article will help you configure the WAN Interface. To start, select Load Balance in the Router configuration Utility.
I hope this helps and if you have any additional questions please feel free to contact me.
Sincerely,
Kevin Yen