Firewall / Port Question

When I raised the firewall request for allowing the clocking machines we have used our external IP and started with port 3734 and added one to the port for each clocking machine.

But I have realised that when traffic is coming in from outside our network the traffic to the clocking machine is using the same port number to try and connect to the clocking machine over the LAN.

So for example if you connect on external IP and port 3735.

The rule on the firewall knows which clocking machine to send the traffic to (Based on the port number) but the traffic is been sent on port 3735 but the clocking machine is expecting traffic on 3734 so the connection fails.

As a quick fix can we change the port that the clocking machine is listening on for connections?