Read this, it works very well
http://forum.mikrotik.com/t/l2tp-vpn-can-not-connect-on-windows-10/131292/1
Another solution is to modify Windows client registry:
http://woshub.com/l2tp-ipsec-vpn-server-behind/
Original MS article about this solution (works also on latest Windows versions)
https://support.microsoft.com/en-us/help/926179/how-to-configure-an-l2tp-ipsec-server-behind-a-nat-t-device-in-windows