OK, thanks for the info. No guarantees that this will work with a modem/router that has been set up to function as a modem only, but here is how to try things out.
Open AirPort Utility
Click on the Main Extreme, then click Edit in the smaller window
Click on the Network tab on the next window
Click on the + (plus) button under the Port Settings window
Description Something like Remote Restart
Public UDP 8888
Public TCP 8888
Private IP Address 10.0.0.1
Private UDP 548
Public UDP 548
Save and Update settings
Restart your Mac
Connect the Mac to the remote network and make sure that Internet is working
Open AirPort Utility
Click on the File Menu at the very top of the screen
Click Configure Other
Type in the Public IP address 217.xxx.xx.xxx
Type in the Admin Password for the AirPort Extreme
That will get you into AirPort Utility for the AirPort
Click on the Base Station menu at the very top of the screen.....up where the File, Window, Help menus are located
Click Restart