I am sorry to know about the issue you have.
Please note that Intel Wireless Adapters are meant to be professionally installed by Original Equipment Manufacturer (OEM) technicians on certified systems only.
Your system is currently certified with the FCC in compliance with the International Special Committee for Radio Interference standards for radiated and conducted electromagnetic interference, installing any other adapter might void this certification, apart from the fact that a different adapter might not work correctly.
What we recommend and encourage is contacting the Original Equipment Manufacturer (OEM) to determine which product is approved for use in your computer.
Only your system manufacturer is entitled to provide the list of validated and certified adapters that are compatible with your system and the proper means to do the hardware exchange for you.
Please refer to the following document.
Regulatory information regarding hardware installation or upgrade
If I am not authorized to install this adapter, why is it for sale on Amazon.com? I work in IT and have no problems installing hardware components.
I need an answer as to why I cannot always connect to the 5GHz network, not regulations about the card.
Some third party companies may take their own decision to sell Intel® wireless adapters but Intel intends to distribute them to Original Equipment Manufacturers only. Therefore, the OEM or the seller should be providing the warranty and the party integrating the device is responsible for such integration.
I also noticed you are using a desktop system; in this case we may suggest the Intel® Centrino Wireless-N 2200 for Desktop or Intel® Centrino Advanced-N 6205 for Desktop which are retail products and are expected to work in a desktop computer.
I know it's been quite a while but I just came across your question because I was having the same issue. I solved it by changing the encryption setting for the wireless N network from TKIP to TKIP+AES. Not sure why but after I did that everything clicked and started working.
I hope this helps!
It is actually better to use AES alone if possible, sometimes those settings can be changed in Windows* as well as in the AP. The article below has some information about 802.11n and TKIP.