1. In NPS, expand "Policies" and select "Connection Request Policies" 
2. In the middle panel, select "TS GATEWAY AUTHORIZATION POLICY" 
   
   
   
   
3. Right click the Properties of "TS GATEWAY AUTHORIZATION POLICY", select "Settings" tab. 
4. In left panel, select the "Authentication".
5. Select "Forward requests to the following remote RADIUS server group for authentication".
   In the dropdown list, select the server group which was configured earlier, e.g. "TS GATEWAY SERVER GROUP".
   
   
   
   
6. Click "Apply" to save the changes.