Problem solved...
Although, some questions has riced...
But to solve it:
iptables -t nat -A prerouting_wan -p tcp --dport <your desired port> -j DNAT --to <the desired computers IP>:<what port to forward to>
iptables -A forwarding_wan -p tcp --dport <your desired port> -d <the desired computers IP> -j ACCEPT
that did the trick...
But my question...
in this line i have put tcp... can i use : to add udp as well (tcp:udp) or do i need a second line for udp?
and
For a port range, like 14000 - 14050 do i put 14000:15050 in the line, do i have to put all 50 lines in the routing table or is there another way to set a portrange in the routing table?