SSL VPN over UDP still attempts to connect to the VPN server on port 443, but unlike HTTPS traffic that uses TCP as a transport protocol, it uses UDP. Some firewalls and proxies may flag this as suspicious and drop the traffic. It can also be seen as peer-to-peer traffic (which it actually is) and again be dropped.
How do I configure the SSL-VPN feature for use with Navigate to the SSL VPN | Server Settings page. Click on the Red Bubble for WAN, it should become Green. This indicates that SSL VPN Connections will be allowed on the WAN Zone. Set the SSL VPN Port, and Domain as desired. NOTE: The SSL VPN port will be needed when connecting using Mobile Connect and NetExtender unless the port number is 443 What ports should be open to let FortiClient make an Aug 22, 2015 What ports are used by your VPN service? - Knowledgebase
Port Forwarding TCP port 443 on DrayTek SSL VPN Routers
Aug 22, 2015 What ports are used by your VPN service? - Knowledgebase
The SSL VPN will prompt for a username and password upon connecting to the VPN interface to authenticate the user. Users can authenticate with Active Directory Credentials (assuming an active AD bind) or credentials set up in a Local Directory.
OpenVPN is a SSL-based solution that can penetrate firewalls since most firewalls open the outbound TCP port that 443 SSL uses. IKEv2 VPN. IKEv2 VPN is a standards-based IPsec VPN solution that uses outbound UDP ports 500 and 4500 and IP protocol no. 50. Firewalls do not always open these ports, so there is a possibility of IKEv2 VPN not being Barracuda SSL VPN Indicator Lights, Ports, and Connectors