If it is a mapped drive under windows:
<a href="https://wordpress.stackexchange.com/questions/249806/file:///networkdrive/folder1/folder2/folder3/DB - Databases">DB - Databases</a>
If it is not a mapped dirve:
<a href="https://wordpress.stackexchange.com/questions/249806/file://///server/folder1/folder2/folder3/DB - Databases">DB - Databases</a>
You should use the 3 /// for the file protocol and 2 // for the network address.
More and more browsers seem to block links with the file:// scheme on pages from http:// and https:// protocols. This used to work (years ago) on some browsers but Chrome and others don’t accept them anymore. For a possible workaround see this answer on SO: https://stackoverflow.com/a/31547188/4021004
Your NAS should be able to make folders accessible via http:// using WebDav, see the Synology KB.