I guess there has been some changes in our web UI, as this is not how I configured Wifi WAN. You can try this steps:
1. Go to Network>Wireless , then click scan network
2. Click Join network on the SSID that you wan to connect, enter the authentification and network name (any name the user prefer) and select WAN as the firewall zone.
3. If the authentification is correct, there should be a new interface in Network>Interface with the same name that you input in previous steps