My home office computer is running Core. I have a laptop running the client software. Both are Windows 10. The client computer does not show itself as a device under Settings > Audio. It only shows the Core and my Chromecast devices. I do not have this problem on my Android phone and tablet, which can play music on themselves or on any other Roon endpoint.
Try turning off any firewalls on the client and the core machines. If that works then enter firewall exceptions for Roon, RoonServer, and RAATserver.