ROON/HQP/NAA/ Ethernet mystery

I know this is not specifically a Roon problem but each time I have posted to this community I have been amazed at the prompt and thorough help I have received here so… my latest issue.

System config… Roon and HQP on MBP i7, 2.3 GHz ,8G ram. Airport extreme wifi router streaming to Airport express as Ethernet bridge to 2010 core duo Mac Mini (headless) wifi on Mini off. NAA on mini, Schiit Multibit Bifrost . This works but I get occasional dropouts. So I decided to run an Ethernet cable from MBP to Mini. My intended scheme was (is) MBP Ethernet to A Extreme… new Ethernet from router to Mini NAA, usb to Schitt. In my situation running the new cable is not a trivial task so after receiving a new Blue Jeans 25’ cable I decided to run a quick test. I connected new cable directly from MBP to Mini, opened screen sharing… all good. Opened HQP prefs and no NAA detected. Closed and reopened NAA on Mini…no joy. Closed and reopened HQP on MBP, nuttin. Hot swapped A express/ Ethernet to Mini ( i.e. back to “normal” and HQP saw my Dac as before. Re-inserted Ethernet MBP to Mini…Dac gone. Restarted mini re initiated NAA all with same result.
So the only thing I can come up with is that the only difference is that the main router (A-Extreme) was out of the cable loop. IOW dose the router need to be in the loop? I can’t easily test this theory cuz Modem/router are on a different floor from Mini. Doesn’t quite make sense to me but I’m certainly no networking guru… Hope someone out there is, cuz I am baffled.
Thanks in advance for your thoughts!

Let me express what I think you said. The hopeful new configuration is:
MBP (running Roon & HQP) [ethernet] Airport Extreme [ethernet] Mac Mini (Running NAA) [USB] Schiit DAC.

But your quick test is
MBP (running Roon & HQP) [ethernet] Mac Mini (Running NAA) [USB] Schiit DAC.

If this is the case then you are probably losing your IP address because they are being served by the router. Try setting static IPS on the two Macs and see if that helps.

Yes, by plugging an ethernet cable between MBP and Mini, you have probably created a completely new sub-net with different IP addresses entirely. If indeed the connection works at all. Your local network cables should all connect back to wherever the DHCP is operating, usually your router, which forms the gateway to your LAN. Then you can fix IP addresses if you so wish, but they need to be on the same sub-net as the rest of the LAN.

Yes sir you got it.
If your response implies that the router must in fact be part of the chain via a wired connection for this to work then my suspicion was correct. Process of elimination led me to that conclusion but I had no clue as to the reason, or how to resolve it." Ignorance is NOT bliss"…so Thank you for helping a networking neophyte!!

I will do as you suggest to test stability before I go through the effort to string the cable between the two systems. If that works the I’ll break out the tools.

Thanks again!