My issue was similar where I could not find the Apple TV when trying to mirror, the icon in the top bar of my mac would appear when the Apple TV was on the network (I tested this by taking it on and off) but there was no option to mirror with it. As well Quicktime could not see the Apple TV but iTunes could and was able to play music and movies.
At first the download was making no progress for the update, when I cancelled it and tried again it would then tell me to try again later. When I restarted the apple TV (did this twice) both times the exact same thing happened. There is no way to connect a Micro USB to the Apple TV so I have no idea what all these people are talking about.
In order to solve the issue I required a reset of the system. Once I reset the system I had to manually tell it to update, even though the factory setting it update automatically. Once I did this it took 10 min to download the update. Once the update was completed the mirroring function and Quicktime could see the Apple TV again.