You may need to use a more advanced router than that one in order to configure it properly. What you want to do could be done, but I don't know that model well enough to say what it is capable of doing.
If you can add DD-WRT or other advanced firmware to your router, it would be able to do it, BUT I can't find any indication your router can use it, so you may be stuck with replacing it or something.
I figured... I have indeed tried to find custom firmware for it, and what can I say? Even googling the router's name is a pain, always another shows up (mostly a wireless one, and this model is wired only).
would it be possible to place the web server into the DMZ of your router?
just make sure you have a good firewall on the web server.
I didn't want to resort to that, but, here I go.... and that did work perfectly.
Create a new virtual server entry
That's what I had been doing all along, since, I haven't found in this model any "port forwarding". Just, virtual server (and port triggering which has never worked for anything