The switch should not make any difference unless it is faulty.
The ip address and subnet masks all seem to be correctly configured.
I would not move the Roon Server to WiFi for anything other than the briefest of tests. Having the Roon Server attached by WiFi is known to be problematic.
Does your router have any settings for filtering multicast packets and igmp packets in particular? If so the filtering should be enabled (to allow such packets to propagate around the network).
Your original post suggests that everything was working until about two weeks ago. Did anything happen about two weeks ago that may have caused this. One culprit that has caused a lot of traffic on this forum is the upgrade of iMacs to Mac OS 15.1 (Sequoia). Did that happen about two weeks ago?
If your iMac(s) have recently been upgraded to MacOS 15.1, try the solution posted at: