Intermittent Connectivity with Windows Server 2012 R2

Good evening,
I’ve a server with Windows Server 2012 R2 installed and 3 client (a Windows 7, a Windows 10 and MacOS laptop).
In the last period I’ve a lot of problems to have a stable Remote Desktop connection between the clients and the server.
I can use the RD for some minutes than I have a disconnection, than I have to wait some time before a new, and short, connection.
I’ve tried to reinstall ZeroTier client, to update it, to restart the network router, and after that I usually I have a small window of “stable connection”.
In all of it, the ZeroTier Central shows, 99% of the time, the server connected to the ZeroTier network.
There is something I can do to fix it?
I’m running out of ideas :.

Best regards
Giacomo