Alright this thread got my interest since I started having some similar issues with my home setup and thought I would share my findings about extending wireless network with the new addition of a 6th Gen AExt .
First I have the following devices all with the latest firmwares (7.7.3 and 7.6.4).
2x AExt 5th gen
AExt 6th gen (AC)
AExp 1st gen (N)
AExp 2nd gen
I ran all my experiments alternating between 2.4GHz band (N) and 5GHz band (N). I only have a couple of devices using the AC band.
I have a wired ethernet network allowing me to run different tests.
Extending AExt 6th gen:
2.4GHz all the devices work.
5GHz band all work except for the AExt 5th Generation.
Extending AExt 5th gen
All devices work for both frequency bands.
Extending Aexp 1st gen. Since it is not bi-band and I had no interest in the 2.4GHz band I only tested at 5GHz.
All devices extend correctly. I did not test the AExt 6th gen as an extender to this device though.
Extending Aexp 2nd gen.
All devices work on both bands.
My devices never go crazy and drop connection like others have reported. When failure occurs, it just won't connect at all. The only devices which don't talk to one another are the AExt 6th and 5th gen when the 5th gen tries to join or extend the 6th gen in the 5GHz band. I am suspecting that it is due to the AC mode but why none of the other devices have this problem???
I was going nuts trying to get my AExt 5th generation to extend my network and could not understand why it would not work. I had my setup extend on 5GHz only and it would rather join one of my AExp which was already extending my network but with a low signal or an AExt 5th gen with an even lower signal. I ended up giving up and switching to the 2.4GHz network on that one Aext 5th gen which in turns expands on both bands. Of course I could just remove the specific name for the 5GHz band, I tried but it made no difference. The 5th gen Aext join the 6th gen only on the 2.4GHz band and all the other devices on the 5GHz band.
One alternative would be to flip the locations of the 5th and 6th gen AExt but I wanted the 6th to be on the wired network and the main unit at the center since it has the largest range...
The way the testing was conducted was under the following conditions for those who are interested:
I set my network by creating a wireless network with one name and then assigning a different name to the 5Ghz band. This gives me control over which band I will be joining/extending. I then set my extending device as either joining or extending a wireless network alternating between the 2.4GHz and the 5GHz network. My network is also access controlled and I have all my devices have one another's MAC addresses for both bands. All devices use WPA2 personal.