AFAIK the WNDR3700 does not support ipv6 out of the box which is a shame because its D-Link cousin does support ipv6 and even has a dedicated page for it in its administration web application. So the hardware should support it.
I just got my WNDR3700 and it freezes after a couple of minutes...
You also need a special receiver on the notebook you are using that is capable to do multi-band wifi.
By the way.. 15MBps is 120Mbps. What is your client adapter and its capabilities?