I was working on a DD-WRT firmware’d router and could not get the port forwarding to work.
Anyway, the solution is as follows: In the Control Panel click on “Administration” then on “Commands”. Copy and paste this inside the Commands Sheet:
iptables -t nat -A POSTROUTING -j MASQUERADE
Click on “Save Firewall” and then “Run Commands”
I found it on dd-wrt.com’s community forums (link) by mojud.