Exactly the same situation.
L2TP server: Windows Server 2019.
Cliente: Windows 10 clients works, Mikrotik works, but Teltonica TRB140 doesn't.
FW I've tryed: R_00.02.06.1, TRB1_R_00.07.00.1 and TRB1_R_00.07.00
L2TP doesn't work with or without IPSEC. When I connect Mikrotik to LAN as L2TP client, everything is OK. But for Teltonica itself no-way.