Yea no firewall rules seem to be in place.
install iftop on the server, try to access it from outside your DMZ and LAN, filter on port 80 (if it's not SSL, 443 if the site is SSL), and watch see if you see the server receive the request. If you don't see the traffic, then the request isn't getting from the Router to the Debian server. If you do see it then you know it is. Also note if it appears that the server is replying. IT's possible this would be a routing issue, or something else, nothing I can think of in the Apache settings would cause the issue you are having. Let us know the results and we can go from there.
