ANSWER: To configure your Apple Airport to work with your new cable modem, you must first reset the Apple Airport to its factory default settings, and then run the Airport Utility.
Because there is no single standard process to reset Apple Airport devices to their factory default settings, you will have to check with your owner's manual for the proper procedure to reset your model of Apple Airport. If you have settings you don't want to lose, make sure to follow instructions on how to save your settings before you reset your device.
You can usually find the Apple Airport utility in your MAC OS under Applications, Utilities. If you do not have the owner's manual, you can find the manual for the exact model number of your Apple Airport on line, or call Apple tech support. ( 800-275-2273)