AirPort Express and ssh
I think you're trying to set up port forwarding, to allow yourself to ssh in from outside, and changing the port to keep it slightly more secure? You don't need to change the port used internally (inside your network) - you just need to change the port used from outside of your network. Then, set up a rule that passes connections on the desired port to the internal. You would for example tell the router that incoming connections on port 1234 should be redirected inside your network to the machine at 10.0.1.123 on port 22. There are some instructions here that might help. (I don't have an AirPort but have set up port forwarding for multiple routers - the basic idea is the same for all)

Asking your computer to accept incoming ssh on another port is more complicated, and (as of El Capitan) apparently requires deactivation of SIP, which is why it may simply be easier to set the AirPort to forward to port 22 by default.