Orbi Router with direct connected devices (mega problems)

I have a new Orbi RBK853 system (wifi6) - sorry but the model dropdown did not find this.  Anyway, its 1 router and 2 satellites.  Setup over the weekend, no drama.  Everything is great.  Super fast wifi everywhere.  Running in AP mode as my ISP provided router cannot be bridged, so it is providing DHCP - so this directly mirrors my setup with Apple Airport Extremes.  So no problems...except...

I have a Mac Mini directly connected to the router via Ethernet.  No problem, can see it and access it.  But certain software that I run on the Mac Mini is causing all kinds of issues.  I run Roon, which is a multicast network music player - and the whole point is that different endpoints (different computers, IOS devices) can connect to the Roon Core running on the Mac Mini.  But this is not working.  When a roon endpoint tries to connect to the Roon core, it gets to the Router, but the router does not let it past.  It even shows trying to connect to mini at IP address of router - but the mini sits behind behind the router.  IOS devices seem to work fine.

Now, for fun I tried unplugging the mac mini from its Ethernet connection and ran it on wifi - and success.  I can connect.  Just for fun, I stuck an unmanaged switch between the mac mini and the Orbi router, and this also works. 

So I have to ask the question, after spending €1000 on a new wifi system, do I need to put an unmanaged switch in between a computer and a router to get it to work?  Only thing I can think is that the Orbi firewall (which I have no control over) is doing something even in AP mode (when it should not because my ISP router is firewalled).  

Any help here would be appreciated.  I find it ridiculous that cannot connect a device to an Ethernet port on the back of a router and have it function properly. 

Thanks

Sean