Hello everyone.
I signed up here specifically to share my experience on this issue quoted above, but with the latest ASUS 68U firmware!
As a new owner of an AC68U, I upgraded immediately to the latest ASUS firmware when I received the router, and before actually using it. After setting everything up, I found that the 2.4 GhZ band refused to pass internet connections to any device connected to that band. The 5G side worked fine.
Since the hardware does not allow you to downgrade to a previous firmware after upgrading to a newer version, I had to hope that the problem was related to the router itself, so we sent it back and got a brand new replacement from Amazon.com.
Before I upgraded the firmware this time, I checked the 2.4 band with the FW already installed on the 68U from the factory, and it all worked perfectly fine. I then upgraded to the latest ASUS firmware for the 68U and once again the 2.4 GhZ band glitched out and refused to pass any internet connections to any devices connected to the 2.4. Some devices could not even connect, and those that could displayed a constant value of "1" in the TX window for that IP and no internet connection even though they showed up as being connected to the router's 2.4 band.
Looking for an alternative to try, I discovered Merlin and downloaded the 11/3/15 version for the 68U and immediately after completing the update, my 2.4 GhZ band began functioning normally again, passing all internet traffic to and from all devices connected to it.
Since the latest ASUS FW definitely killed the 2.4 band on two brand new 68U units, I am curious if any of the latest ASUS FW code is being integrated into these Alpha builds?
I was considering giving this Alpha 4 a try myself until I read this post, which precisely describes the behavior I saw with the latest stock firmware on two brand new 68Us.