|It is not your Modem blocking the ports, Modems don't do that, routers do. Have you forwarded port 80 from the router to the web server?|
Failing that it could be your ISP. Port 80 is the default port for Web pages. Some ISPs block port 80 which prevents you from running a Web server. This is to cut down on excessive bandwidth. Port 25 is often blocked as an anti-spam measure.
Your only solution in this case is to to talk to your ISP and ask them to unblock the ports or to change your ISP to one that does allow web servers.