Step 1.
Unfortunately, many routers do not support L2TP connection with IPSec (encryption), so it needs to be disabled on the server side.
Log in into VPNext server settings with SoftEther VPN Server Manager (view manual) and in “IPsec / L2TP Setting” section check “Enable L2TP Server Function (Raw L2TP with No Encryptions)”.
Click “ОК”.