To open the firewall GUI via WAN interface, create a firewall rule to allow remote firewall administration.
Firewall > Rules, under WAN Tab
Action: pass
Interface: WAN
Protocol: TCP
Source: The IP address or subnet of the client, an alias containing management hosts/networks, or (not recommended) Any
Destination: WAN Address
Destination port range: HTTPS
Description: Allow remote management from anywhere (Very Dangerous! – may be used only in lab environments)