RADIUS Authentication for L2TP VPN (Error: No groups have been found for this user)

Question

So I'm trying to configure a RADIUS server to use for authenticating L2TP VPN users from my Sophos UTM 9. I've installed the NPS role on my Windows Server 2016 domain controller and registered it with AD. I then added the UTM as a RADIUS client: 
 
 I then set up a network policy with the following settings: 
 ( no constraints are configured) ( everything here is default) 
 I then added the server to the list of authentication servers on the UTM with the following settings. The server test passes, but the user test fails.

Does anyone know why this isn't working? This is my first hands-on experience with RADIUS so please forgive me if this is a simple problem.

Josh Marchant · Answer

I'm already using that same security group for the SSL VPN. Since posting this I've tested the VPN connection using RADIUS authentication and it seems to be working fine even though the test doesn't work.