You missed one important part of Tray896's comment
"That would be done on the networking side, not the web server side."
You need to configure your firewall to forward requests on port 80 for your external IP address over to the internal IP address of your web server.
Also, creating a www record on your DNS server is only for your network. You need to get with your registrar or whomever holds the records for your external DNS and create the www entry there so that everyone can resolve it.