The L2TP Connection Attempt Failed
Because the security layer encountered a processing error during initial negotiations with the remote computer.
This error pops up when you try to connect to a VPN connection through the L2TP protocol. In this particular case, there is nothing wrong with your Windows, your computer, your network adapter, or the credentials to the L2TP connection—everything is just fine. The problem is your router.
If you are able to bypass the router completely and insert the Ethernet cable that comes through your ISP directly into your PC, and the connection suddenly starts to work, that means there is a setting in your router’s web panel that prevents L2TP connections (and other types of VPN connections) from working.
All you need to do is log in to your router’s control panel. There are quite a few models of routers from different brands, and their interfaces are somewhat different. I will be using a D-Link device as an example, but if you have a different model with a different web interface, you will have to look around for a similar setting.
The name of the setting you have to activate usually has something to do with “pass-through”—something like “VPN Pass-Through.” As soon as you activate it, apply the settings, and reboot the router just in case. After that, the VPN connection should work just fine.