If you are using the same network name (SSID) for both the 2.4GHz and the 5.0GHz networks, the 2.4Ghz "tends" to "overpower" the 5.0GHz network. Try setting the the two networks to different names.
Also, not all wireless adapters, built-in or added on, can do the 5.0GHz network, unless stated directly on the adapter specifications.
And finally, it has been reported the D-Link internal web page will not report the 5.0GHz 'device' connections, due to a firmware bug with the recent firmwares.