How do you allow active directory authentication outside the network?

I have a mobile application that I need to authenticate the user before letting them use a form.  I am querying active directory to authenticate.  This works internally, if the user is on the network, but how do I allow authentication outside of the network with forms authentication. 

When a user tries to access the form outside of the network, the user gets stuck at the login screen.  The user is not being authenticated.  Do I have to allow certain permissions for this to work?  If so, what do I have to do to get this to work?
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