Well, whenever you change the modem that is directly connected to the AirPort Extreme....or...you change the ISP, the AirPort Extreme needs to be reset back to default settings and then set up again.
Make sure that the Ethernet cable from the modem is connected to the "O" port on the back of the AirPort Extreme
Power up the AirPort Extreme and let it run for a minute
Hold in the reset button on the back of the AirPort Extreme for 7-8 seconds and release
Allow a full minute for the AirPort Extreme to restart to a slow blinking amber light status
On your Mac.....
Open AirPort Utility (Finder > Applications > Utilities > AirPort Utility)
Click on the other Wi-Fi Devices button
Click on AirPort Extreme xxxxx
AirPort Utility will suggest the correct set up for the AirPort Extreme
Follow the prompts
You can use the same name for the Base Station and Wireless Network that you used before, as well as the same password that you used before
Click Next and AirPort Utility will configure the AirPort Extreme for you
Should be a green light on the AirPort Extreme after a minute or so
You are done with the set up
You can now open AirPort Utility on the iPhone now, and enter the password if needed.