The task is to install sstp server on linux with radius support (or any other external user management support).
If you can do it, please get in touch with details about yourself and your work.
How many VPN tunnels you plan to use?
Regarding SSTP server on Linux.
At the moment there is no implementation of the SSTP vpn-server on Linux.
Instead, I would recommend using OpenVPN.
Or, to use the device from Mikrotik(it has SSTP-server implementation).
There is also a product SoftEther VPN Server(SSTP) - but it does not support external authentication.
So OpenVPN for you to be the best solution.