These are some basic instructions for getting RADIUS authentication setup between a Sonicwall firewall (TZ-Series) and a Windows Server (2016, 2019, etc).
1. Starting on the WIndows Server side first, open Network Policy Server (under Control Panel > Administrative Tools or Search). Expand “RADIUS Clients and Servers”, and Right-Click on RADIUS Clients and select New. Enter the information as follows and click OK.
2. Next expand “Policies” and Right-Click on Connection Request Policies and select New. Enter the Policy Name, and select “Policy Enabled”. Under Conditions, enter your Friendly Name. 
Under the Settings tab, make sure “Authenticate requests on the server” is selected under the Authentication tab. Click OK.
3. Next, right-click on “Network Policies” and select New. Name the policy, make sure it’s enabled and select Grant access under the Overview tab. Under Conditions, select “Client IPv4 Address” and enter the private IP of the Sonicwall. You can next through until FINISH. Optionally, you can add User Groups for additional security and Authentication Methods.
4. Now heading over to our Sonicwall, assuming SSL-VPN is already installed and configured, go to “Settings” under Users. Make sure “User authentication method” is set to RADIUS + Local Users and select CONFIGURE RADIUS. Under Settings > RADIUS Server, click ADD… and enter your Server’s IP from above. Make sure to open Port 1812 on your AD Server. 
Under the RADIUS Users tab at the top, Select “Use RADIUS Filter-id attribute on RADIUS server” and select “SSVPN Services” for the Default Users Group. Just head over to “Local User & Groups” and make sure that “SSLVPN Services” group has LAN Subnets added under it’s VPN Access tab. Now you can click Test and make sure both Connectivity and Credentials are working. 
Finally, open your SSL VPN Sonicwall Netextender and test connectivity. Make sure the DOMAIN in the client matches the NetBIOS Domain name and that it is also configured under your SSL-VPN > Server Settings on the Firewall. 
These are quick instructions to assist our teams for basic configuration help. We share it here to help others that may be trying to set this up or troubleshoot a failing setup.