Hello All,
Just switched to the Xiaomi AX3600 for speed reasons. This router comes with 3 different wireless interfaces. I have two enabled, one for 2.4ghz and one for 5ghz. On my network I have a RPi with Homebridge installed, which works with mDNS using Avahi or Bonjour. On the 2.4GHz network I can see everything on my Home app which is breadcasted by my homebridge server, but when I switch to 5GHz network, everything is grayed out and I cannot communicate with them.
The weird thing is that I can actually access the devices, ping them, SSH them etc.
The even weirder thing is that on my previous router, I had exactly the same config (other than the actual hardware) and Home app was working on both wireless radios.
I have no isolation, no firewall rules outside of the default ones, and no weird configuration.
Anyone that had the same issue? Any suggestions? 
Update: I think that it is relatable to the fact that the radio1 that I am using is wifi AX, still investigating.