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It seems there's a bug in at least the 7.7.7 and 7.7.8 firmware revisions that causes the AirPort Extreme to become unmanageable (though still functional as a router) if you sign into one of your Apple IDs to use Back to My Mac :/ The AirPort should appear and be manageable (by clicking on it) for a brief period-quickly click on it, click Edit, then clear out any Apple IDs in the 'Back to My Mac' section.It took several tries but eventually, using your approach, I got it to extend the xfi network. Plug it back in, and connect to the WiFi network as soon as possible, then immediately go to the AirPort Utility. Apple, the Apple logo, AirPlay, AirPort Express, Apple TV, FaceTime, iBooks, iPad, iPhone, iPod touch, iSight, iTunes, Mac, and Safari are trademarks of Apple. Use the Airport Utility app on your Mac or iOS device to set up the Airport Express - preferably youd want to connect the WAN port to the Xfinity Gateway and AIrport Utility will automatically set up the AE in Bridge Mode instead of the Gateway.Unplug your AirPort Extreme, and wait 10 seconds.

CONFIGURE APPLE AIRPORT EXTENDER UPDATE
For the latest information on AirPort software, check Software Update in System Preferences or the following Apple website: Apple Support Downloads. AirPort Utility 5.6.1 allows you to configure and manage the new AirPort Express with simultaneous dual band 802.11n. If you've had an AirPort Extreme for a while, and recently (within the past year or two) had it go missing from your network (when you open AirPort Utility you get 'Device Not Found'), there's a good chance you ran into the same issue I did. Download AirPort Utility 5.6.1 for Windows.
