You can get apps that scan wifi that should tell you which frequency they are operating on. Try inssider for windows? Shows 2.4 Ghz and 5Ghz channels in a graph. It might be transmitting N on the 5ghz, but your iphone shouldn't see it.
Is there an option in the router to change this, might be worth a go. Change to wireless N 2.4ghz, or even put down to G and see if that works.
Sometimes the chipset in the router will just be incompatible with your iphone, its daft but ive had it before where my iphone would go really slow connected to certain access points.
also try the usual just incase.
Change the router channel from "auto" to 11.
- On the iPhone: "Forget this network" and attempt to rejoin.
- Reset the iPhone Network configuration (General -> reset -> reset network).
- Rebooted the iPhone 4S. (Also hard reset).
- Connect to an unprotected network to see if your phone connects to a different router.