It is not clear from your post, how your network is set up. I assume, L2TP server is behind router with dst-nat to this server, and you are trying to connect from Windows client. If so, Windows registry modification is required on client computer. Read this (although article is about Windows Vista,...

I am new vpn user and I have configured an L2TP server with a shared key on my laptop. I have tried numerous ways like disabling of firewall but it can’t connect. Anyone can help me.