Cannot commission IKEA Bilresa (Matter/Thread) via iOS — Thread Border Router Required error despite fully working OTBR setup

Configuration Matter/Thread
1

First of all, I made this post mostly via AI (Claude), to make a good summary of all the things tried. I hope this is not a problem. If so, please let me know.

My setup: Home Assistant OS on a VM, hosted by Proxmox. iPhone 17 with iOS 26. Thread border router is a custom ESP32-C6 OpenThread RCP (based on this guide).
Home Assistant version: 2026.6.4 Matter Server version: 9.0.3 OTBR add-on version: 3.0.1

Problem

When trying to commission the IKEA Bilresa (scroll wheel version) to Home Assistant via Matter, the iOS commissioning flow (both via HA Companion app and Apple Home app) fails with: "Thread Border Router Required"

This happens after scanning the QR code, when the white Apple commissioning screen appears showing "Connecting..." for a few minutes before failing.

The OTBR add-on logs show no activity whatsoever during the commissioning attempt; the Bilresa never reaches the border router.

What I have verified and confirmed working

After extensive troubleshooting, the following are all confirmed working:

IPv6 networking

  • UniFi Cloud Gateway Ultra advertises IPv6 fd00:1::/64.
  • Proxmox host was not accepting Router Advertisements due to forwarding=1. Fixed by setting net.ipv6.conf.all.accept_ra=2 and net.ipv6.conf.vmbr0.accept_ra=2.
  • All devices (Proxmox, VM, iPhone) have routable fd00:1:: addresses.

OTBR

  • OTBR add-on starts cleanly, ESP32-C6 RCP communicates correctly via Spinel
  • Thread network forms, OTBR becomes leader: Role detached -> leader
  • Border Agent starts: BorderAgent---: Border Agent start listening on port 49154
  • mDNS announcement is correct and visible on the network:
  avahi-browse -t -r _meshcop._udp
  address = [192.168.0.61]
  address = [fd00:1::d6a5:2c95:aa28:f09f]
  port = [49154]
  tv=1.4.0, nn=ha-thread-8673, ...
  • iPhone can see the OTBR in Discovery DNS-SD browser under _meshcop._udp, with all three addresses including the routable IPv6 address
  • UDP port 49154 is confirmed open and reachable via nmap from Proxmox host:
  49154/udp open unknown

Matter Server

  • Matter Server starts cleanly, Thread dataset is loaded, fabric is configured
  • Thread credentials successfully synced to iPhone via Companion app → Troubleshooting → Sync Thread credentials

Proxmox networking

  • Bridged networking (vmbr0), not NAT
  • Multicast snooping disabled on bridge
  • No firewall rules on UniFi

What still fails

Despite all of the above being correct, iOS commissioning still fails with "Thread Border Router Required". The OTBR logs show zero activity during commissioning - the iPhone never attempts to connect to the border router.

The Apple Home app gives the same error, confirming this is an iOS-level rejection, not a HA issue.

The Matter Server logs show no commissioning activity either — the device never reaches HA.

Anyone has any ideas?