What router? Are the SSIDs different or the same for 2.4 and 5? If different, what's the difference between the original and the new one you created that does work (and why isn't that good enough)? If the same, how are you specifying which band to connect to?

Also, are you trying to force 40mhz on 2.4 or something? Is encryption the same on both? Can you connect if you remove the password altogether?