Whenever you change service providers, or your service provider changes the modem, you will need to reset the AirPort Extreme back to factory default settings and then set it up again to work correctly with the new product.
To perform a factory default reset......
Power off the AirPort Extreme for a minute or two
Hold in the reset button on the back of the AirPort Extreme first, and keep holding it in for an additional 7-8 seconds while simultaneosly plug the power back into the AirPort Extreme
Release the reset button after the hold period and allow a full minute for the AirPort Extreme to restart
The AirPort has now been reset, and it can now be configured to work with the new modem.