I'm having difficulty getting a soap connection working on a port, say 4433
What do I ahve to set in order for this port to remain open?

I have added it to the open ports. Is this all I need to do. The config is otherwise factory default settings.

So, you just want to run a public server for a specific TCP port inside your private network?

You should be able to do that with either NAT >> Port Redirection or NAT >> Open Ports, eg: -

NAT >> Port Redirection
Index No. 1
Enable
Mode: single
Service Name: soap
Protocol: TCP
WAN IP: all
Public Port : 4433
Private IP: 192.168.1.15
Private Port: 4433

Prompts may vary slightly depending on your model / firmware.