Whenever you change the modem or modem/router on a network, the AirPort Extreme will need to be reset back to default settings and then set up again to work correctly with the new modem. To do that you would.......
Power up the AirPort Extreme for a few minutes.......if it is not already on
Hold in the reset button on the back of the AirPort Extreme for 7 to 8 seconds, then release
Allow a full minute for the AirPort Extreme to restart to a slow, blinking amber light status
Insure that an Ethernet cable connects from one of the four LAN <--> Ethernet ports on the modem/router to the WAN "O" port on the AirPort Extreme
To start the setup, click the WiFi icon at the top of your Mac's screen and look for a listing of New AirPort Base Station
Just below that, click directly on AirPort Extreme and then follow the prompts of the setup "wizard"
When you set up the AirPort Extreme again, you can use the same wireless network name and same password that you were using with the AirPort Extreme with the "old" modem if you wish, or use a new wireless network name and password