I have an ASUS RT-AC66U Wireless Access Point. It has the option for a RADIUS server. Does this work the same with Active Directory?
I have Windows Server 2012 R2 DC and it has the LDAP service installed as well. Is that the same thing?
RADIUS and LDAP/AD are two different techniques. RADIUS just defines a protocol for delivering and checking credentials, whereas LDAP/AD are directory services for storing arbitrary data in a tree-like structure. LDAP/AD may be used as data-backend for RADIUS, but they might not be used interchangeably (you might authenticate »against« an LDAP/AD server, but you would have to »speak« RADIUS).
What you want is a RADIUS server that stores/retrieves credentials over LDAP/AD, but you'd still need a dedicated RADIUS implementation. You could try to understand LDAP/AD as the message, but RADIUS as the messenger.
For more information see this.