This all sounds like your problem is not with local disk access, but with your network, which I gather is probably Microsoft's built-in peer-to-peer Windows networking. The first solution that comes to mind is that you could reinstall that network from scratch. An alternative might be for you to install remote control software such as Anydesk, TightVNC, or the newer program RustDesk. That way you could remotely edit files on your upstairs computer from your downstairs computer, or vice versa.
Good luck.