Yes - I think you will need a VPN connection. What that will look like depends on the environment used to host uStore.
Amazon has their "Virtual Private Cloud" and Azure has their "Virtual Network" to enable this sort of thing.
I would start there. I would also engage XMPie as they may have some experience with this sort of configuration.
I'm doing a new install of FFCore on a local machine and we have uStore/XMPIE hosted in a cloud enviroment.
Would I need to establish a VPN in order to cast out the link for FFCore so uStore can access it? Any advice on what that would look like?