https://medium.com/@tsaiid/win10-%E7%84%A1%E6%B3%95%E9%80%A3%E7%B7%9A%E8%87%B3-synology-vpn-server-a4fea4558221

l2tp-passthrough-nat.reg

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\PolicyAgent]
“AssumeUDPEncapsulationContextOnSendRule”=dword:00000002