Then you need a VPN. I have a VPN but it is configured on my router, not on OMV. Pretty sure it could be setup using OMV if you so desired. I see there's an OpenVPN plugin that you can install.
Sorry for the confusion.
Alternatively, if all you want to do is move some files, you could use FTP instead of SMB. Then you wouldn't need the VPN. You would just need to open the appropriate port on your firewall (recommend using something other than 21) and connect to the server from anywhere.
Right I've done some further research into the topic. The OpenVPN just needs some files loaded to it and it should work.
You mentioned you had a system setup although the VPN runs on your router. Once the VPN is configured, do you access the files by putting the IP address into a file explorer ?