Try this: https://aerovisionit.co.uk/pptp-and-l2tp-port-forwarding/ It is also mentioned in the troubleshooting link I shared above.  The only part I think is wrong (or at least I disagree) is needing to port forward UDP/1701.  UDP/1701 is used, but outbound only and should not be opened for unsolicited inbound connections.     The reason I think you need that is because now your L2TP server is behind NAT. ... View more

@BGFernandes How many users to do you have your network, 250mbps should be enough to handle a good amount of client devices before you start seeing issues.  ... View more