Devices published via Homekit not visible outside the home network

Hi there,
a couple of information on my network setup: I have two different VLANs, HOME and IOT (you can never be too careful regarding chinese IOT equipment). My Home Assistant instance is located in the HOME subnet. I have an Apple TV in the IOT subnet. The AppleTV is configured to act as a Home Hub and is shown as “Connected” in the Home.app at all times.

I am currently experimenting a bit with Homekit, currently just exporting a simple temperature sensor. I have already added Home Assistant as a Homekit Bridge to the Home.app which works fine whenever i am at home.

Problem is: The sensor is shown as “Not responding” when i am outside my home (or simpy deactivating wifi on my phone). I was hoping that the AppleTV would simply forward its value to the iCloud where my not-at-home devices could pick it up. Apparently this is not the case.

I searched a bit around the internet, and saw a post where someone suggested to enable an mDNS repeater between both subnets, so i enabled it. I can tell it is working because the Sonos App on my phone in HOME is able to discover the Sonos Players in IOT. Unfortunately this does not help to see my sensors from outside my home.

Of course i enabled all the necessary firewall settings. IOT devices are able to connect to HOME via 5353/udp which is for Bonjour protocol. HOME devices are able to connect everywhere, so they should be able to connect to IOT via Bonjour as well. I also have opened the 21063/tcp port from IOT to HOME which is Homekit’s own port.

What am i missing here? I mean, although my network setup is slightly more complicated than “No VLANs at all”, it is not that much complicated. I strongly assume that some Avahi autodiscovery voodoo is not properly configured, but i cannot confirm that, and do for now not know how to properly set up, since i do not know what to set up at all.

Are there some checks i can fulfill, like “is HA responding on homeassistant.local” or something like that?