Has your router got an option to configure and internal IP as a DMZ? Is so then that will let ALL traffic through to the internal server on your private LAN network. Your external port scan indicates that port 80 is open but that doesn't say whether that is port 80 on the router or the one forwarded to on the internal server.
When you configure the webserver, tell it to listen on the INTERNAL LAN IP and on your linux desktop add "192.168.xx.xx yourdomainname.tld" to /etc/hosts to force your browser to go directly to the webserver on the LAN IP.
Also, when testing your DNS, set all your TTL settings to 300 seconds so any changes propagate within about 5 to 10 minutes.