All other devices works well with while Windows 10 always complains:
In this post you will learn how to resolve L2TP VPN connection using Windows 10 or other Windows operating
system. If you would like to view other posts than please check our blog page for other helpful posts
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, Router 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.
Windows 10 error message doesn’t contain string Error 809.
Details from microsoft https://blogs.technet.microsoft.com/isablog/2010/08/04/more-than-one-l2tp-vpn-connection-from-behind-a-nat-device-fails-with-error-809-when-tmg-2010-has-been-configured-as-a-vpn-server/
Recommend to add following to L2TP/IPsec Windows instructions html page
resolution to L2TP VPN connection used in Windows 10 by so many users and would help to find how to resolve L2TP VPN connection on Windows
Client behind NAT devices
Solution: Modern Windows devices do not support L2TP/IPsec connections when the Windows computer or VPN server are located behind a NAT. If the Windows VPN client fails with Error 809 when trying to establish a VPN connection to an MX located behind a NAT, add the “AssumeUDPEncapsulationContextOnSendRule” DWORD value to the Windows registry. This DWORD value allows Windows to establish security associations when both the VPN server and the Windows based VPN client computer are behind NAT devices.
For Windows XP:
Data Value: 2
For Windows Vista, 7, 8, 10, and 2008 Server:
Data Value: 2 Note that after creating this key you will need to reboot the machine