In this video, we are going to configure dual WAN in failover mode with mwan3 on OpenWRT Luci.
Assign the new WANB interface to the existing wan firewall zone. Under Advanced Settings, set a metric value of 20, as this WAN connection should be secondary and not used by default.Configuring WAN failover.

Setting up WAN Failover is a lot easier than it sounds, so let's do that. Step 1: Select your Site.Click on the WAN 1 interface's gear icon in order to configure the interface. A small window will pop up, which you can use to set up the WAN Failover interface.

Such details provide a deeper understanding and appreciation for Configuring Wan Settings For Failover.
1. WAN Failover Setup Configuration: Go to Basic Network > WAN & Uplink > Physical Interface Tab Setup Multi-WAN Configuration, here is ISP Ethernet as WAN-1, Cellular as WAN-2 for example.

In the Network Failover Multiwan Interface section, configure the failover rules for both connections as follows: To begin, click on the pencil icon associated with your WAN connection and establish the rule that will activate the failover mechanism.
Steps to Configure Wan Failover. On Network -> Basic -> Interfaces page, configure the chosen port for WAN connectivity. Provide a name to this port and configure the IP addresses.Configure WAN failover settings as shown above.