I have the same issue where my Sgs 5 and Sgs 7 were not able to see the 5GHz band of my Dlink travel router. It saw every other router's 5GHz band, but for some reason didn't like that router, makes me think something in the wifi chipset that Samsung uses does not like that particular configuration of 5GHz.
I forgot what I did to fix it, but it was on the router side, I think I set it to a specific channel rather than auto, or set it to 20/40 channel width, I can't remember.
Other than that, make sure the 2ghz and 5ghz bands have different ssid's, or you may be connecting without knowing it.