What region are you located?
What Mfr and model # is the mais how router?
What wireless modes are you using on 5Ghz?
What security modes are you using on 5Ghz? Temporarily disable WPA security on the 5Ghz radio and see if the adapter will connect to the router. Re-enable after testing.
Does the Adapter work with other 5Ghz routers, say at a friend or families place?