Not sure this is your issue, but there is a small checkbox at the top of the first page after Join. You need to uncheck this in Luci or it will delete all your wireless connections.
Below that is “Name of the new network” which should be WWAN (but may be WAN which I think is standard)
Below that choose WAN for the “Create / Assign firewall-zone”
The check box can really much up the works. This post the author offers a GUI fix to disable the checkbox by default, but I have not tried it (but really should).
https://forum.openwrt.org/viewtopic.php?pid=278230#p278230