There are a couple of things to check, both on the pc and the router.
On the pc, click Start, then type this command in the Search bar and hit enter:
This will brind up the device manager. Click the Arrow next to network adapters, then right click your 5100 AGN and select Properties. Click the Advanced tab and make sure 802.11n is enabled.
Now the router...
Use the computer directly plugged into the router via cable and open a web browser, then enter this address;
A login screen will appear:
Once logged in, click Wireless Settings at the top left side. There are settings for both the 2.4 Ghz and 5 Ghz networks on this page.
On the bottom half of the page, 5 Ghz, make sure SSID is enabled. Make sure up to 300 Mbps is selected.
Save the settings - is this enough to get your connection?
Keep in mind the router is broadcasting both 2.4 and 5 Ghz, so they will look like two separate networks - the default names were NETGEAR (2.4) and NETGEAR - 5G (5). You may need to click your network icon on the bottom right corner of your desktop to pick up the second network and connect to it.