Will the traffic be directed to/from your WAN machine through a gateway server? If so, before you make an iptables rule, you could test network connectivity and what ports are involved by using an ssh tunnel to port-forward the traffic from the remote port 5900 to your local machine (Computer A) at some arbitrary local port through the gateway server (you will need a login to that server however).
If you can open up a putty or other ssh terminal on the destination machine you can type:
ssh -L LocalPort:RemoteHostIP:RemoteHostPort username@GatewayHost -N
Like: ssh -L 10000:192.168.12.104:5590 myuser@gateway -N
This will forward traffic from 192.168.12.104:5590 through the gateway to your local machine's port 10000. Perhaps this is not applicable for you though. I often have to deal with jumping through a gateway, hence this is useful for me.
Last edited by jnielsen7; 02-08-2013 at 05:25 PM.