this generally fails and you have to add a registry entry
The network connection between your computer and the VPN server could not be established because the remote server is not responding. This could be because one of the network devices (e.g, firewalls, NAT, routers, etc) between your computer and the remote server is not configured to allow VPN connections. Please contact your Administrator or your service provider to determine which device may be causing the problem.
By default, Windows L2TP/IPsec VPN client doesn't support connections to VPN servers located behind NAT devices. To enable NAT Traversal (NAT-T) support, make following registry change
Navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\PolicyAgent
Create a new DWORD 32 type value:
- 0 No connection to servers behind NAT (Default).
- 1 Connection where VPN server is behind NAT.
- 2 Connection where VPN server and client are behind NAT.
Hope this helps