My ISP's web interface is blocking OS X Server?
If your AirPort doesn't go to another router, but goes straight to the modem or whatever you have for internet access, check your public IP online and your WAN IP in AirPort Utility.

Then, if they match, make sure to forward port 80 (unless you prefer to specify a port every time you go to your site) and test if that wasn't already the case.

If the IP's don't match or if the issue persists, check with your ISP.