Sharing how I did it - all within the Luci interface - this has been working without issue for more than a month. You should make a backup of your configuration before making any changes.
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Go to Network > Interfaces and select the Devices tab. Click configure on the br-lan device. Remove a LAN port from the br-lan “bridge ports” option by selecting the menu and unchecking a switch port such as “lan1”. This port will become it’s own WAN port. NOTE - ETH2 … is labeled “LAN 1” … the ports are: WAN=ETH0, LAN2=ETH1, LAN2=ETH1. I removed ETH2 for the following steps. Save/Apply these changes to remove the selected LAN port from the br-lan bridge.
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While still on the Devices page, scroll down and click the “Add device configuration” then select ETH2.
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Go to Network > Interfaces and “Add new interface” Give the interface a name such as “wanb” (lower case, no numbers … it will show up as all CAPS). NOTE - I used ETH2 from above and protocol DCHP Client, (Interface name “wanb” (it will show up on Interface tab as all caps - “WANB”). For device select the LAN port you removed from br-lan earlier. Select whatever protocol is required for this interface DHCP, PPPoE etc. Go to DHCP Server tab - check “Ignore interface”). Assign the new interface to the wan firewall zone
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Apply any remaining changes.
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Test the new interface - use ping to check/verify each connection.
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Set up mwan3 - Interface, Member, Policy, Rules to include the new “wanb” interface - there are many tutorials and guides for this - you will have to configure all the settings. When you set up the ‘tracking addresses’ if you change the order of the addresses from those of the WAN port you will be able to identify this new interface in the GL.iNET page Network>MultiWAN.
This will show up on the GL.inNET Admin page - Network>MultiWAN as “Ethernet” … if you changed the order of tracking hostnames you will be able to identify this new WAN connection.
Best of Luck.