Window Switch uses multicast DNS (aka Zeroconf, mDNS, bonjour, ..) to advertise itself on local networks.
This makes it possible to automatically connect to servers without any prior configuration. This feature can be disabled (both on server and client) if desired.
To enable this feature, you may need to install extra packages - please refer to your operating system's vendor for details. You should ensure that your firewall allows inbound and outbound access on port 5353.
If your server is detected when it starts, but not when it has already been started then more than likely mDNS is not resolving correctly: check your operating system's firewall on both client and server.