It sounds like this is an issue with your surroundings. It doesn't seem to show the characteristics of a faulty card.
In general the 5GHz band isn't going to be great for distance use with a wireless router. If you are trying to connect with the router outside of your house, then it's probably not going to show up. 5GHz doesn't travel well through walls either, so that could be an issue if you have lots of walls in your house. Having a weak signal could also mean that your house paint or materials in the house may be bad for letting the 5GHz signal through.
If none of those apply, then there could be a problem with your router. I would try fiddling with the settings on the router a bit and see what happens. If it's still under warranty call the manufacturer, and they may be able to hook you up with a solution. Another thing to try would be to connect to another 5GHz network if possible. If the problem still persists I would try resetting all network settings (settings>general>reset>reset network settings), bearing in mind that this will delete all networks that you have connected to, and require you to put in all of your wireless network passwords again. Don't worry about doing so though, it won't delete any data (music, videos, books, etc.) or any other settings that don't pertain to wireless network settings.
If none of the above solutions work for you, as a last resort you could either call up AppleCare, or head into the nearest Apple Store, and they should be able to help you out.
Hope this helps,