@AGTess 

Hi, the IOT network and Main Network are not separated. So Apple HomePods could still voice control and communicate the smart devices on the IOT network.

Compared to wireless standard routers, the IOT network is like the 2.4GHz band, and the main network works like the 5GHz band. They might use different SSIDs but they are under the same LAN network.

When the internet is down on the Deco, the physical Wi-Fi networks are still active and under the same LAN network. You should be able to control Smart Devices with Voice Assistant Hub (Homepods).

Thank you very much and best regards.

  @AGTess in short, the IoT network is still able to communicate with the main network so your Homepods should still work with your TV, phones, etc.

Device isolation is useful if you have something with an internet connection that you suspect might be vulnerable to attack like smart switches or lightbulbs etc, so you can provide them with an internet connection to function normally but you are able to shield your local network from them by preventing local connections to/from it. All the isolated devices are put into the same "pool" so to speak so provided you isolate all the devices you want to be able to communicate locally it shouldn't matter if they are on the main SSID or the IoT SSID. The only network that is completely separate from the main one is the Guest network.

With regards to local control during an internet outage, that depends on the devices rather than the Decos, if they support local communication then you should be fine as long as the Deco doesn't get stuck in a loop searching for a WAN connection which is apparently a common problem that might be resolved but I haven't had an opportunity to test it myself.

Hope this helps!