If you add this to your firewall, it will allow port 80 from the WAN interface.
config rule
option enabled '1'
option target 'ACCEPT'
option src 'wan'
option proto 'tcp'
option dest_port '80'
option name 'AllowWANWeb'
You can also do this through the LuCI web interface by adding a traffic rule under the "Open ports on router" section (TCP port 80).
Be aware, though, that this is not necessarily a good idea -- it exposes your router's web interface to the internet which may invite hacking attempts and such. Make sure you have a very strong password if you do open this port.