Before playing LDN multiplayer, go to Options > Settings > System and ensure "Enable Guest Internet Access" is OFF and "Enable Vsync" is ON, then head to Multiplayer settings and ensure the mode is set to "Ryujinx Ldn" if you want to play online, or "ldn_mitm" if you want to play with others in the same local network. Once the emulator enters local wireless, a Windows firewall warning will appear. Make sure to allow it to connect.