While connecting Plex to Soundiiz; you can at the first login step specify the "IP:PORT" configuration (or leave it empty) and a "Server Name" (can also be empty to select the default server); which, could be very useful if your Plex login has access to multiple Plex servers.
Be sure to fill in the proper server name in this "Server Name" section before moving to the following steps, which in the Plex.tv login step.