I think it may be long past time for a sticky that says:
Don't use port 80 - many ISP's block it or use it for remote management of your Gateway device.
Assign a fixed IP address for your server in your router.
Assign the same address to the NIC on the server.
Typically, you will want to assign an address at the top of the address range so that it will not be interfered with by DHCP. e.g. 192.168.1.254
Port numbers 8080, 8081, 8082, ... usually work very well.
Once set up, test access with
http://192.168.1.254:8080
Then test
http://servername:8080
Then test
http://servername.subsonic.org
Ironically, (as always), doing it the hard way, IS the easy way.