How is your webserwer port on 192.168.0.4 host? If you have also port 80 comunication is imposible.

So will start fiddling around with Open-WRT So, you can work with openwrt on this router, I recommend Gargoyle.

Hence my question, if there is any trick to fool the original firmware into using one of those ports, or if a more flexible firmware from another model might do the trick on the 743ND No you can't....

Give us screenshot from this setting. Do you have public IP address on WAN router?

Can you see my post number #5? TFTP application you search original firmware, type in address and password your router and click upgrade.

You have to download TFTP and original firmware, next you have to upload this on router by this application. Everything is in tutorial.