Not only your DAC is missing, Settings|Audio doesn’t show your Core at all. All 4 audio devices from your list should show up there as ‘Connected to Core’ (between ‘Connected to this Mac’ and ‘Networked’).
You should check your firewall(s) and network settings (multicast IGMP).
There is also a HQPlayer zone shown in Roon. If the HQPlayer runs on your Antipodes to control your DAC, you should use that zone to play music. From HQPlayer Setup:
If you’re using a device in HQPlayer, make sure that the device is disabled in Roon’s audio settings. Both Roon and HQPlayer expect to have exclusive access to the device, and they might conflict if you configure both at the same time.
Can you please check to see what the behavior is like with the wifi extender out of the mix? If you try connecting both Antipodes directly to the router, is there any change in behavior? What is the specific model of the router you have? Do you have “enable multicast routing” checked in the settings?