It's not recommended to open up a website (Dynamics CRM) externally outside our network using AD authentication (windows authentication). For these scenarios claims based authentication such as ADFS is recommended. Refer the blog below:-
http://blog.webfortis.com/microsoft-dynamics-crm-authentication-on-premises
Hope it helps!
Regards, Abhishek Bakshi If you find this post helpful then please Vote as Helpful and Mark As Answer. Check my blog on https://mydynamicscrmblog.wordpress.com/