A few users (one Reddit user as example) across different smart-home platforms have run into a really odd issue that may also affect SmartThings: certain ISPs provide IPv6 Prefix Delegation that doesn’t renew or route correctly - and some Asus ZenWiFi firmware versions don’t handle this gracefully. The result is a “half-working” IPv6 stack that quietly breaks multicast, Thread traffic, Matter onboarding and even normal app loading on Wi-Fi.
The symptoms look completely random: devices drop offline in waves, Matter commissioning fails without explanation, voice assistants respond after several seconds, or apps on 5 GHz just stop loading images. Everything feels unstable even though the network looks fine at first glance.
One workaround that has proven effective is to disable the ISP’s DHCP-PD on the Asus router, set IPv6 to Native/Stateless, and use a local ULA prefix (for example fd00::/64) - see the linked thread for details. This keeps the local IPv6 environment that Thread and Matter rely on, but avoids the problematic delegated prefix entirely.
It won’t apply to everyone, but if you’re using Asus AiMesh and your smart-home setup has been behaving unpredictably lately, this is worth checking. For some users, it resolved weeks of instability overnight.