Matter devices have wrong IPv6 addresses

Configuration Matter/Thread
1

Hello!
I’m using a laptop with HA OS to run my HomeAssistant installation.

I recently installed AdGuard Home on HomeAssistant. Unfortunately my router does not allow me to configure the DNS server that is advertised via DHCP. Thats why I wanted to disable the router DHCP server and use my own.

Doing this using HA requires me to set a static IP address for the HA installation.
Unfortunately, once I set a static IPv6 address on my HA installation, all Matter devices are unreachable.
Switching back to Automatic will allow them to connect again.
While having it set to static I also reset one device and tried reconnecting, but unfortunately the app couldn’t set up the device at all.

I noticed that my HA is running with these addresses (anonymized):
global: 1111:1111:1111:5900:c15f:3b23:57bc:73e4/64
link-local: fe80::ec17:3891:7369:f41a/64

My matter devices though have addresses as such:
1111:1111:1111:5910:7606:42aa:bd9d:f821

I noticed that they are placed in an entirely different network than my HA installation and have no link-local address.

They are connected using Thread, my Border Router is an Apple TV.

EDIT: I found out that my devices have a network route to this different network via the AppleTV. I wonder why the Apple TV creates this new network?

Can anyone explain why the devices are placed in this different network and how I can resolve my issue?

Thanks a lot!
Kind Regards,
Daniel

2

In general, the Thread Border Router is an IPv6 Router. The Thread devices on the wireless mesh side use one IPv6 prefix (chosen by the TBR), and the LAN side of the Thread Border Router uses another IPv6 prefix.
The TBR wants to create the LAN side prefix and advertise it over the LAN so that HA and others can self create a complete IPv6 address using this prefix. The TBR however will not create nor advertise this prefix if it sees another router on the LAN side advertising say a GUA, in which case the TBR will use the GUA prefix on the LAN side instead.

The TBR also advertises a route on the LAN side so that others including HA can use the route to reach the devices on the wireless mesh.

So probably what is happening, is that when you turn off HA’s automatic IPv6 setting, it does not pick up the advertised route from the TBR.

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