Windows 10 client can't connect to VPN

Server Version 5.1.5 running on Mac OS X 10.11.5. I've turned on VPN and can successfully connect from Mac clients.

Trying to connect from a client running Windows 10, and am getting an error ("connection between your computer and the VPN server could not be established"). Since "mobileconfig" files are not recognized by Windows, I used the following manual settings:

VPN provider: "Windows (built-in)"

Connection name: [a descriptive name]

Server name or address: [my server -- verified that I can connect to web pages using this host name]

VPN type: "L2TP/IPsec with pre-shared key"

Pre-shared key: [the "Shared Secret" as displayed by the VPN panel of the Server app]

Type of sign-in info: "User name and password"

User name, Password: [login credentials, as set up in Server]

I tried following the instructions here:

OS X Server: How to connect to VPN service from Windows - Apple Support

But those instructions don't mention Windows 10 (or 9, or 8), so I'm not confident it will help; and I can't complete the "Change the Local Security Policy" instructions, because secpol.msc doesn't seem to exist -- possibly because this is the Home Edition.

Is this a common problem, or something unique to my setup? Any recommendations? (It shouldn't be this hard, right?)