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1. Click the User & Device section in the left navigation panel and navigate to User → User Groups.
2. If you have an existing user group, click on it to edit its settings. If you don't yet have a user group, click Create New to create one.

3. On the Edit User Group or New User Group page, enter the following information:

   
   

    

   Name	SSL VPN with 2FA
   Type	Firewall

   
   

4. Click the Create New button in the Remote groups section and select the DualShield RADIUS remote server. You do not have to specify a group.

   
   
   
   
5. Click the OK button to save the user group settings.

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1. Connect to the appliance CLI. Consult the documentation that accompanied your Fortinet device for more information.
2. Execute the following commands:
   #config

# config system global

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    set remoteauthtimeout 60
end

# config user radius
    edit <RADIUS Server>
        set timeout 60
end

Reference: https://community.fortinet.com/t5/FortiGate/Troubleshooting-Tip-Users-randomly-fail-to-connect-to-SSLVPN/ta-p/189823