new modem airport express cannot be found via airport utility

There is an old network saying that goes something like this......

Make just one change on a network.......and everything else on the network will change.

You will need to reset the AirPort Express back to factory default settings and then set it up again, so that it can recognize the new modem and be programmed with the correct settings for the new modem.

To reset the AirPort Express,.....assuming that it is already powered up......hold in the reset button on the back of the AirPort Express for 7-8 seconds and release. Allow a full minute for the AirPort Express to restart to a slow, blinking amber light status. That is your signal that the Express has been reset and can now be reconfigured again, just like you did the first time that you installed the Express.

If you need more help, we'll need to get the make and model number of the modem to be able to provide detailed, step by step instructions.

I did reset the Express, but it still does not show up in Utility.

If you did reset the Express.......then it will not show up in AirPort Utility as it did before you changed the modem .....because AirPort Utility displays AirPorts that already have been set up correctly......and at this time, the AirPort Express has not.

You need to first setup the Express to work with the new modem/router.

You do that by first clicking the WiFi icon at the top of the screen, then clicking directly on AirPort Express under the heading of New AirPort Base Station

I assume that you have an Ethernet cable connecting from one of the LAN <--> ports on the NV 589 to the WAN "O" port on the AirPort Express.

When you click on AirPort Express, that will get the setup "wizard" running so that you can reconfigure the AirPort Express.