Set the Identity source of your network administration policy to Active directory rather than Internal. (Probably called 'AD1'). If you want to use _both_ then create an Identity Store Sequence first, that checks internal followed by AD, and use that...

I worked around a similar constraint by having our AD admins create the computer object with the name of the ACS system, then give my AD account rights over that object. Then I was able to join ACS to AD using my own AD credentials.