Unifi & Honeywell Lyric T6 pro connecting to HomeAssistant (HomeKit or Resideo)

Installation
1

I am having problems with my Honeywell Lyric T6 pro thermostats (total of 12) in a system that is about 1.5 years old. I am using a UDM with a fresh factory reset, dedicated IOT vlan and only the 2.4GHz for the IOT SSID. The thermostats are assigned a static IP address but after rebooting most of them shift to an IP address out of the DHCP range. I am having difficulty controlling the thermostat from a HomeAssistant installation and I am currently thinking that it has something to with the wifi connection of the thermostats.

In the beginning, I was using a USG and the thermostats worked fine connecting to a dedicated IOT SSID on its own vlan using the Resideo iOS app… I also use HomeAssistant. I initially connected the thermostats to HomeAssistant using the Resideo integration and everything worked fine to control the thermostats with automations. At some point, I moved the thermostats to connect to HomeAssistant using the HomeKit controller integration in order to have everything local instead of going through the cloud of the Resideo app. At some later point I upgraded the USG to a UDM.

A while back, every now and then one of the thermostats would show as unavailable in HomeAssistant even though I could ping it on the network. I would reboot the thermostat, and things would go back to normal. Over time, the frequency of the thermostats dropping from HomeAssistant became more and more frequent. Out of frustration, I tried switching back to the Resideo application. That helped a little, but the problem continued.

The past few weeks, the thermostats have been totally uncontrollable from HomeAssistant using either integration. Out of frustration, I tried a couple of troubleshooting steps:

  1. I set up a separate brand new installation of HomeAssistant (HassOS on ESXi) to connect the thermostats thinking that I had made a change in the settings that was causing the problem - no change in behavior

  2. I did a factory reset on the UDM to go back to the default network setup thinking that I had made a change in the settings that was causing the problem - no change in behavior

  3. I can control the thermostats in the Resideo app itself, but not HomeAssistant using either the HomeKit or Resideo integration

  4. I have repaired the thermostats to Wifi multiple times, with no change