I do not really understand how you have connected your system but I think it is:
1.( I am guessing as you don’t actuallly say) Router to Bartok by Ethernet connection . However which of the two Bartok ethernet inputs have you used? Note that one of the two, “Loop”,is not currently supported and is there for a future use yet to be disclosed ( see page 32 of User Manual).
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A second ethernet cable connects Bartok to Aurender. However you have already used up the only operational ethernet port in 1. above. So how is this connected?
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Aurender is also connected to Bartok by USB cable. However you have said that you are connecting them by ethernet ( see 2. above). Why have you connected Bartok and Aurender twice?
Which Aurender device have you got? Many of them are described by Aurender as Servers as they are streamers with built in storage. I am assuming that they use a form of UPnP for serving as otherwise they would not be compatible with other devices.
As I said I do not know which Aurender device you have. So there are more than a few unknowns to me here. However I can see that the Aurender has an ethernet port and I am therefore, as above, assuming that it supports UPnP. Why can you not attach the Aurender to the router and also the Bartok ( use Main ethernet port, not Loop). Then things should work as intended. ( subject to the UPnP server software installed which is another topic for another time).
If you use the USB output from Aurender then it is not acting as a UPnP source but as a USB streamer in which case control and navigation would be by using Aurender Conductor app and not Mosaic as then Bartok would be working as a DAC only. Therefore as then the Bartok’s network board would not be in use you wouldn’t see much information with Mosaic.