There are several solutions available, depening on your exact needs.
As I understand, you do not have a server machine for NAS, just plain file server. In this case for free solution to keeping files in sync you might adapt a distributed kind of version control system (VCS), which does not require a server, such as Git.
There is some overhead to using a VCS compared to some kind of specialized backup/sync software, however, there are some upsides too, which might be useful to you, depending on your exact needs (keeping info on previous versions of files, for example).