Note: this will move your Broadband Router’s web-configuration to an unexpected port :8080, instead of the :80 your browser expects.
Management | Access Control | Services then disable the WAN side HTTP service (why would you even expose this?), change the port for the LAN side to the Alternate HTTP port of 8080, and hit
Advanced Setup | NAT | Virtual Servers and hit Add. Select the correct interface, fill in the other details including the Web Server’s LAN address, ensure you’ve got
Status: Enabled for the port forwarding, and hit
Run up a trial HTTP server using something like
sudo python -m SimpleHTTPServer 80
and check for access from outside. Kill the server, because that isn’t safe for production use.