@pilipili this is an old thread. The solution for this has been discussed several times. I have dozens of Windows 10 users connecting via L2TP. There is a setting on the client computer that must be made. This is not unusual, some other L2TP servers require the same.
Windows Control Panel > Network and Sharing > Change Adapter Settings
Right click the VPN connection name and go to Properties
Under Data encryption check “allow these protocols” and make sure MS-CHAPv2 is checked.
With this your connection will work fine. There are issues with public and hotel wifi often blocking the ports used for L2TP but that is not a Peplink or client issue. For those scenarios PPP will usually work but of course is not recommended.