To enable port forwarding on your DD-WRT router after connecting it to OpenVPN you can add firewall rules under "Save Firewall" in the Command section of the DD-WRT interface.  

For instance this rule will accept incoming packets on port 80 and forward them to 10.1.1.107

iptables -t nat -I PREROUTING -i tun0 -p tcp --dport 80 -j DNAT --to-destination 10.1.1.107
iptables -I FORWARD -p tcp -d 10.1.1.107 --dport 80 -j ACCEPT

Once you save the firewall reboot the router.

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