Hi Jay Jay Action,
According to your description, I understand that you use pure SFB on premise, is that right?
For two factor authentication, it provides improved security by requiring users to provide two forms of authentication or identification, namely a user name/password combination and a token or certificate.
Based on my research, I couldn’t find a method to do two factor authentication without block external user for mobile. I will share some documents about two factor authentication
https://technet.microsoft.com/en-us/library/dn308562.aspx
https://technet.microsoft.com/en-us/library/dn338071.aspx
https://technet.microsoft.com/en-us/library/dn308571.aspx
Hope this reply is helpful to you.
Regards,
Alice Wang
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