The first thing is to find out if the option to Enable Remote Setup over WAN was selected when the AirPort Extreme was originally set up.
Open AirPort Utility on the Mac
Click the AirPort Utility menu in the upper left corner of the screen
Click Configure Other
in the Address field, enter the WAN IP address
In the Password field, enter the device password for the AirPort Extreme
If the AirPort Extreme setup screens appear, you are in. If the setup screens do not appear, then someone will need to connect to the AirPort Extreme locally on the same network to enable the Setup Over WAN setting under the Base Station menu.
Even if you have the correct settings, some firewalls will block connections like this. Not much that you can do about that other than try connecting from another network.