I struggled with this today on a Windows 10 computer. What solved it for me was set up a new rule to allow incoming connections in the advanced firewall settings.
I also had port forwarding set up on the router, but haven't tested to see if it still works if I remove it. I know setting up the port forwarding alone didn't work, but I don't know if allowing incoming connections is sufficient on its own.
@Laura190 You probably don't want to port forward. Port forwarding is for when you want to broadcast a server over the internet. In OpenLP's case, we normally just want to share the server within the local network.
On windows you can change your network type from "public" to "private" if you want to allow incoming connections on the local network. (no firewall rules required)