How to get access from the wifi and from behind the router to the file server?

Question

Windows

I have a windows computer that needs to access a file server.
The problem is that the computer sees the world through wifi, so the IP address cannot be done directly.
The scheme is as follows: Computer -> public wifi (mine, but without a password and everything else) -> wifi router -> router -> central router on Linux, looking at the Internet and the third interface to the LAN with the server -> the desired server.
What's the easiest way to do it? Raise openvpn on linux?
Of course, you can not give access to the file server for everyone.