WNDR3400v2 - Problem with 2.4Ghz connections

I experienced the same problem a few days ago:

Can connect to 5Ghz w/out any problems.

2.4 Ghz shows up with a strong signal on my android phone, Win 7 desktop, Samsung phone, Macbook and iPad.  However, when anything tries to connect, the signal immediately drops and no connection is made.

I seemed to have fixed the problem by doing the following:

1. I enabled the 2.4 Ghz guest network.  It was disabled before.  Once I did this, I am able to connect to the main 2.4Ghz using WPA2-PSK.

2. After connection to the main 2.4, I disabled the 2.4 Guest Network.  Immediately, I lost my connection to the main 2.4 channel.

3. I re-enabled the 2.4 Guest Network and immediately I was reconnected to the main PW protected 2.4

So, if you did not have the 2.4Guest Network enabled, enable it and see if that works.  It doesn't matter if you the Guest Network has a password or not.  In may case, I used WPA2-PSK on both the regular and Guest network.  FWIW, I do NOT have the 5Ghz Guest Network enabled.

As others have noted, if you remove security from the regular 2.4Ghz channel, it will connect.  However, I want to use P-word security.

Hope this helps!