If you host your Azure MFA User Portal outside of your MFA Server e.g. in a DMZ , the User portal server has to speak to this MFA server via SSL using the SDK and also via a certificate. Make sure the SSL of you MFA server is valid. It might be self signed internally on that server ( cannot be from the domain as the DMZ server can’t speak to your domain ) so you need to create a new one and transfer it securely to the User portal and install it on the Trusted Root Authority 

System.Security.Authentication

This certificate cannot be verified up to a trusted certification