Huh, that seems to have fixed it! Thanks. It seems the firewall was checked for "private" but not "public". I checked both boxes and now we can connect.

I have two computers on a LAN. They can see each other and connect in any other game that I've tried (usually by direct local IP) and they can see each other in Windows to transfer files between each other. I downloaded the zip-file of Factorio experimental 17.1 directly from the website and extract...