I know the problem, its the NAT-T
Do you have the L2TP Server installed in the Windows Server ?
The Windows Server is behind a NAT, or with Public IP ?
If you are using just the Mitrastar like core If you are using the Mitrastar as a router core, I would advise you to acquire a small router that can perform the VPN function, thus freeing resources both the Mitrastar router and the Windows Server
For just $ 40 you could have a team powerful enough to perform all these tasks
If you want to keep the Mitrastar, we could put the router bridge mode, it can also be a known problem in the registry
More than 4 years working as a network specialist
Resolution of problems of any level
Designed and set up large networks in hotels, shopping centers, businesses, etc.
Management of IPSec VPN servers, routing & switching, hotspots, load balances, Firewalls, BGP, MPLs
Management and total control in manufacturers such as Mikrotik, Fortinet, Clavister, Ubiquiti, Ruckus, Mojo, CISCO, Sophos, HP, Netgear