What are the dropbox alternatives for running on a server with no open sessions?

Question

Data storage

Good afternoon.
There are two servers on Windows Server 2008 R2 in two different cities.
Internet connection at points leaves much to be desired.
It is necessary to deploy some more native analogue of dropbox on the servers, which would run inside the server without the presence of sessions and provide one directory for general access with the ability to resume data and save their state when working offline until the next connection to the Internet.
Dropbox works very "crutch". In other words, it does not work at all if there are no working sessions on at least one of the machines.
PS Initially, an FTP server was raised, but due to constant failures in the Internet, the mutual transfer of a couple of gigabytes of data per day by the servers turned out to be impossible .

Answer 1

Alternatively: https://syncthing.net/
Run as a service independent of user login