Did you ever determine or get a reply to this situation? I just got a ASUS EA-N66R extender for my ASUS RT-AC68U router and on the router it is set 3600, the Extender it is 0.
Although it recommends NOT to; I have intentionally used the same SSID for the 2.4 range as I want devices to roam across and connect seamlessly between the EA-N66R in AP mode (hard connected to our LAN) and my router which in on a upper deck. I am thinking this setting should be identical as all other settings I have mirrored (aside from keeping channels in AUTO mode, which I think I may need to lock them into the same channels, but one thing at a time).