Solved a massive HomeKit and Matter stability issue by fixing local IPv6 on the router
I want to share the solution that finally fixed weeks of unstable behavior in my HomeKit and Aqara setup. This advice applies to any router and any ISP because the root cause is not one company. It is the combination of an unstable IPv6 prefix from the ISP and a router firmware that does not handle that prefix correctly. When this happens the router can break the internal IPv6 stack even if IPv6 looks enabled.
HomeKit and Matter rely heavily on local IPv6. All communication between hubs Thread border routers Thread devices and HomeKit accessories happens inside the LAN. Siri on the Aqara S1 Plus uses the same mechanisms. None of this requires global IPv6 from the ISP. Local IPv6 is the important part.
What caused the chaos for me was an inconsistent IPv6 Prefix Delegation from the ISP. When the router tried to use the unstable prefix it created packet loss multicast failures and mDNS problems. The result was predictable but confusing
- Matter onboarding failed half the time.
- Thread devices went to no response.
- The S1 Plus Siri was slow or did not answer.
- HomeKit accessories disappeared and came back.
- Apps on five gigahertz did not load images.
- The network looked fine but behaved terribly.
The breakthrough came when I stopped using the ISP provided IPv6 prefix completely and kept only local IPv6 active. This gives HomeKit and Aqara what they need and removes the part that caused the instability.
Here is the exact fix that worked
Enable IPv6 on the router but turn off DHCP prefix delegation. Set IPv6 mode to Native or Automatic. Use Stateless mode so clients get link local IPv6. Add a simple local IPv6 prefix for the LAN, for example
- LAN IPv6 Address: fd00::1
- LAN IPv6 Prefix: fd00::
- Prefix Length: sixty four
After applying this the network became instantly stable
- The Aqara S1 Plus Siri responds immediately.
- Matter devices pair correctly every time.
- Thread stays healthy with no random dropouts.
- HomeKit latency dropped to almost nothing.
- All five gigahertz apps load perfectly again.
This works because local IPv6 remains active and stable, while the router no longer accepts the unstable external IPv6 prefix. IPv4 continues to handle internet traffic, while IPv6 takes care of internal smart home communication exactly as it should.
If anyone has questions about this setup or wants help adapting it to their own router, feel free to ask. I am happy to share settings or walk through the process step by step.
In the world of smart homes every problem has a solution, and sometimes the smallest network change unlocks the biggest improvement.