I do not have an Airport Extreme, but some googling did find some documentation about port forwarding: https://discussions.apple.com/docs/DOC-3415
Section 2 is talks about the port forwarding configuration, but don’t skip section 1 to define a static address.
Here is Section 2:
2. Setup Port Mapping on the base station.
AirPort Utility > Select the base station > Edit > Network tab
Click the "+" (Add) button under Port Mapping.
Description:
Public UDP Port(s):
Public TCP Port(s):
Private IP Address:
Private UDP Port(s):
Private TCP Port(s):
Click "Save"My Notes:
– You can Ignore the UDP ports, since we are talking TCP to the device.
– The Public TCP port is the port you use externally such as on the canyouseeme.org site.
– The private IP and TCP Ports is what you use to talk to it on your local network.