Help connecting IKEA BILRESA dimmer switch

I’m using two ZBT-1, for a Zigbee network that should be parallel to a Thread/Matter network.

The second ZBT-1 has been able to be configured to the Thread firmware, however I wasn’t able to pair a BILRESA dimmer switch to the OpenThread border router.
When I scan the QR code with the Home Assistant app on Android, I’m told that this device would need a border router, which I’ve already set up with the second ZBT-1 stick.
I would be able to pair this device in the Google Home app to the cloud, but that’s not the Home Assistant server.

IPv6 is active, home assistant is using a static IPv4 connection, but has IPv6 IPs listed in the network settings.