port mapping? simple Q

I’ve read everything but i cant find an answer to this simple question.

I want to port map: request comes in to port 2600 the router fowards that to a machine behind the NAT (192.168.5.2) on port 161.

I cant get it to work. here is the nat rule i’m trying:

ip firewall nat add chain=dstnat protocol=udp dst-port=2600 action=dst-nat to-addresses=192.168.5.2 to-ports=161


does not work. please help me i’ve read so much and cant find the answer to this simple Question.

tks