As you are changing the config - WebUI will not like it and nothing will show up in WAN placeholders.
But there is a workaround with Load Balancing.
You should head over to /etc/config/
and edit the network config file so you would have two Wired WAN interfaces. One that you have now and another one identical but with a VLAN option added.
This way you will avoid Load Balancing drop-down menu breaking and you should be able to select your modified WAN interface.
To your newly created WAN interface give this option: