You don't need to configure Plex in the WiiM app in order to use Symfonium, but the fact the Ultra is discovered means mDNS is working.Yes. I forgot, do I need to select Plex in my Media Services list?
Are other apps able to detect Chromecast - assuming it's enabled? To test UPnP discovery you could try BubbleUPnP.
As much as I hate to suggest it, have you tried rebooting the Ultra?