Thank you have just done that.
And... in doing that, I realised this:
The Apple TV client was connecting to the macmini (DVR)'s wifi despite it having an ethernet interface. When I tried to force the Apple TV client to connect via the DVR's ethernet IP, it says IP not available.
If I disable wifi on the macmini, then the client automatically connects via ethernet and the issues that I was experiencing dont occur.
Why it wasn't working as well via wifi is a separate issue and am not fussed about that.
Why it won't connect via the DVR's ethernet interface is the question. Taking into account that other mobile clients connect without issue, so its only the Apple TV that wouldn't.
Already tried: restarting the Apple TV, the network and reinstalling the app etc.