Hello Aditya,
The URL input in Fiori client should be
http(s)://<FronEndServer>:<Port>/sap/bc/ui5_ui5/ui2/ushell/shells/abap/FioriLaunchpad.html?sap-client=<Client>
Please check in VPN environment, the hostname or IP address of your frontend server can be resolved or not?
Regards,
Ning