Whenever you change the modem or gateway to which an AirPort connects, or you change Internet Service Providers (ISPs), it is always a good idea to reset the AirPort back to factory default settings and then set it up again so that it will pick up all of the correct settings that it needs from the new modem.
If you had called Apple Support for help, this is the first thing that they would ask you to do, so if you have not already done this, it would be a good general troubleshooting step to perform. It might also be a good idea to swap out the Ethernet cable between the modem and AirPort as well.
If you decide to do this and it does not solve the issue, then we'll need more information from you.......
The make and model number of the modem
Which model of AirPort Extreme that you have
Approximately how old the AirPort Extreme might be
Whether you are using a Mac, a PC, or an iPhone / iPad to change settings on the AirPort