I have no experience using wired ethernet at home, but here are some other thoughts...
- In Whitebear DMRA, cancel out of the initial discovery window after it finishes. Then check the box in upper right hand corner "Use Windows Discovery", then tap "Select a Media Renderer..." to repeat the discovery search. I never found this necessary, but maybe give it a try.
- For further internal checks, in MC left panel, have a look at:
Services & Plug-ins > Reporter > Net
Services & Plug-ins > Media Network > ...
You should see 5 servers running and may glean some Info from each of them, including their Activity Logs.
In particular, the Device Discovery Server should be pretty active. Confirm that its listed Port (typically 1900) is open through your firewall.
- Another external-looking-in check (but using Wi-Fi), assuming both JRiver Media Center 28 Service and JRiver Media Center 28 windows program are running:
Is the MC server (and therefore library) discoverable by any phone player apps (MC Panel, JRemote2, MO 4Media, BubbleUPnP App, etc.)?
(BTW I find the BubbleUPnP App is particularly good at discovering all renderers on the network, independent of MC. It also provides lots of detailed info on discovered renderers)
- Windows checks
Control Panel > All Control Panel Items > Network and Sharing Center > Advanced sharing settings
Private > Network discovery > Turn on network discovery