I just purchased an iPhone 5 and had a similar issue with my WNDR3700v2 router. I did try many of the suggestions listed in Apple and NetGear support and finally got everything to work.
My resolution path included:
- Upgraded Firmware from 184.108.40.206 to 220.127.116.11 (yes, it gives you warnings - but my upgrade went smoothly and the interface is much, much nicer)
- Changed SSIDs for the 2 and 5 GHz sides (make them different so your devices have to rediscover and authenticate to "new" networks)
- Set the 5GHz encryption to WPA-PSK [TKIP] + WPA2-PSK [AES]
This process let me connect my 5 and reconnect my other devices without having to lower my security settings.