Go into the wireless settings, and change the "Mode" to the highest available for both the 2.4 GHz network and the 5 GHz network.
From the manual for your router: "Up to 150 Mbps is the default and allows 802.11n and 802.11g wireless devices to join the network. G & B supports up to 54 Mbps. Up to 65 Mbps supports up to 65 Mbps."
If you want more detail, go to pages 30 and 31 of the WNDR4000 manual: »www.downloads.netgear.com/files/···ar11.pdf