Rescue PC

L2TP/IPsec VPN from Windows 10

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

Open regedit.exe
Navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\PolicyAgent
Create a new DWORD 32 type value:
Name: AssumeUDPEncapsulationContextOnSendRule
Data: 2

• 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.

reboot

Hope this helps
Cheers
Rescue PC

This actually doesn't help for me on windows 11. I run my own VPN server on my new DS920+ and as well on my old NAS but get same result.

You would need to post more information on what VPN what protocols, and what errors you are seeing