CMAK (Connection Manager Addministration Kit) is a feature part of the windows server operating system that enables you to create custom app for use by your VPN remote users. After creating it, they can run it on there client system to connect the office after in which they have connection to the internet first.
CMAK configuration is of a few steps which i have shown in the video below.
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See also, How to Configure VPN with NPS on SSTP, Server 2012