With ecobee, there’s another route you can try: via HomeKit (the protocol).
Since it is HomeKit, communications and at least basic controls would be local. However this is rather convoluted and thus not for everyone.
- Set ecobee to pair via HomeKit, to one of those HomeKit hubs by Apple (iPad, apple TV, etc.) This is temporary.
- From the HomeKit app, remove ecobee from Apple’s HomeKit garden, so that to force the ecobee into a “hey I’m a HomeKit device, any HomeKit hub out there wishing to pair with me?” status.
- Setup your HA with the integration " HomeKit Controller", so that to make your HA instance a (non-apple) HomeKit hub
(The “ecobee” integration is not what you are looking for. It works, and works great, but not local.)
- Now pair the ecobee with HA via the “HomeKit Controller” integration. HA should now pull ecobee into its fold.
(Do not use the other “HomeKit” integration, which is to push what you have inside HA into another HomeKit hub from Apple, and thus not what you want)
Now, your HA should be able to talk to the ecobee via HomeKit (the protocol). The ecobee unit itself would still talk to ecobee cloud, to get weather data to display on the ecobee unit, for example, but at least HA gets to control the thermostat locally, even when your internet is down.
There you go. As I said, rather convoluted and thus not for everyone.