Try telnet to router (if you use Oleg's Firmware) and try something like this (16000 is Port, 192.168.1.2 - IP of PC in your internal area)
iptables -I FORWARD -p tcp --dport 16000 -j ACCEPT
iptables -t nat -A PREROUTING -i vlan1 -p tcp --dport 16000 -j DNAT --to-destination 192.168.1.2