Seems like this is a fairly common problem. Despite my AP supporting both 2.4GHz and 5GHz, the Surface Pro 3 I’m testing for work point blank refuses to connect over 5. Which is a problem seeing as my neighbours wireless security camera pretty much floods 2.4 making it next to useless.
Anyways, from what I can gather it looks like there’s a problem with the Surface’s Marvell wireless NIC driver that means if your AP is using the same SSID for both networks the driver ignores the 5GHz one. I’ve just changed the settings on mine to use a different SSID for each and the Surface connects fine. It’s worth a try if you’re having the same problem.