On the network firewall, I opened up port 80 and forwarded it to the Windows Server 2008 R2 running IIS 7.5. Port 80 is open in the Windows firewall.
On the LAN, the website is accessible without any trouble. When accessed from the outside, I can access it only 3 times every few hours before access to it cuts out from the outside and the browser displays the error message "cannot display webpage".
There aren't any access limits in IIS or the network's firewall as far as I can tell.
IIS is running PHP 5.3. The default website has been deleted. The bindings for the website's domain are set for mydomain.com *:80 and mydomain.com 192.168.0.3:80
[edit]: Just to check if there was some problem with the firewall or ISP in using port 80, I set up the firewall with port forwarding for 8081 externally to 80 for the LAN. The domain/website/page was still inaccessible by a browser outside of the network.
[Edit2]: The DNS entry for the domain resolves to the correct external IP. If I attempt to access the website via the external IP, it loads three times, then ceases to be accessible with the browser message "cannot display the webpage".
Best Answer
The IP address was already in use by another device (which was undocumented). The device was also secured against ping requests and port scans, so it appeared as though there was nothing there. I only found out about it when I called the ISP and they looked at the ARP table in the modem and saw a MAC address (which was unknown to me) assigned to the IP address.