Hi James,
I will try and describe the process as much as I can for you and anybody else struggling with routers.
First in Filemaker set your IWP port to 591. This is assigned to Filemaker and will stop any conflicts with other ports. Look in the IWP and note the IP address. This is the Local network (LAN) address, try it from your web browser, if you get the Filemaker publishing page then you are publishing but only on your LAN
Next you need to get your web static address connecting through the router to your computer with IWP running. The IP address on your LAN will continually change as it is decided by the router using DHCP. In your routers configuration pages find DHCP and set the range of the last 3 numbers of the LAN IP address range from 100 to 255, this gives you use of 1 to 100. Next choose one of these numbers for your IWP computer and find port forwarding (called virtual server in my d514) in your router and set the external port 591 to connect to internal port 591 on the IP number you have chosen for your computer. For example mine is 198.168.0.8 because I chose 8 as the fixed address.
Now you need to set your IWP computer IP address by manually configuring it to the one you chose, mine is set to the number above 198.168.0.8. You will no longer be connecting to the LAN using DHCP but with a fixed address the router will know that this is where to send port 591 traffic. Can't help much with static addresses on windows as we use Macs and I haven't played with windows for years.
Try connecting with another computer from outside of your network, dial up is useful here. Use the static IP address provided by your ISP in its number form first with :591 at the end. When you get that connecting then divert your domain name.
Try this link for more info http://www.portforward.com/default.htm.
Colin