I'd check two things.
Firstly, using a separate computer on the local side of your network, can you access IWP on your host computer using a web browser? This is just to check that the firewall on the host computer is not getting in the way.
Once you have this working, you can move onto the router. I'm not familiar with this particular router, but I guess you must be able to set up a DMZ host or some kind of static routing so your router can pass requests for the IWP port to the host? You say you can reach your Apache server so this suggests that port 80 at least is being routed correctly. One thing you could try is to stop your Apache server temporarily, reset the Filemaker IWP client to work on the standard port 80, and see if you can reach it from outside. If this works, and the first step works, then you know the issue must be with port routing.
Once you've got this working, please reactivate the firewall on the host computer with just the appropriate ports opened. Having a DMZ host with no firewall is a recipe for disaster for the security of your host server.
This useful blog post from SixFriedRice explains the ports that need to be opened. http://sixfriedrice.com/wp/filemaker-firewall/
Hope this Helps