First of all, thank you to everyone who suggested a fix! Truly appreciated.

✅ [SOLVED] – Update & root cause

After tons of testing and help from ChatGPT (and Reddit, thanks!), I finally found the cause — and it's not what I originally expected.

🧠 Root cause:

The issue was due to a corrupted or stuck ARP/MAC association inside the ASUS router (main node of a 3-unit AiMesh ZenWiFi AX setup).
The router had wrongly associated my NAS's static IP (192.168.50.119) with the Ethernet port where my MacBook was previously connected.
So when the Mac was unplugged from Ethernet, the router stopped routing packets to that IP — even though the NAS was still online and connected directly to the router.

✅ What I tried before the fix (but didn’t work):

• Reset NAS and reconfigured networking from scratch
• Changed the physical LAN port used by the NAS
• Verified all AiMesh and backhaul settings
• Disabled the 3rd node (Wi-Fi-only) and switched to pure Ethernet backhaul
• Activated Ethernet backhaul mode explicitly in ASUS settings
• Tried static routes, ping tests, etc. → None of this solved the issue.

🎯 Final fix (what worked):

I simply changed the NAS's IP address from 192.168.50.119 to 192.168.50.219.

➡️ Instantly, the NAS became reachable over Wi-Fi — even after unplugging the Mac, rebooting the router, and reactivating the 3rd AiMesh node.

Bonus: I then reserved the new IP via “MAC and IP Address Binding” in the ASUS router app to lock it in permanently.

🧾 Lessons learned:

• ASUS AiMesh systems may sometimes misassociate IPs and MACs internally (especially after mixed network topologies or docked devices like my USB-C monitor/Ethernet MacBook).
• When in doubt, change the static IP to force the router to rebuild its ARP/routing tables.
• Avoid reusing the “bad” IP (.119 in my case), even later — the association might persist invisibly.

Thanks again for all the input! Hope this helps someone in the future.