Matter controllers find devices using mDNS — multicast DNS on your LAN. Some routers throttle or block multicast under load, on guest networks, or via “optimization” toggles. Symptom: accessories never appear during setup though the vendor app can see them on Bluetooth.
Spot throttling
Discovery fails on Wi-Fi but works if you hotspot the phone briefly (test only). Eero, UniFi, ISP gateways with “smart queue” or IGMP snooping cause this. Many devices fail at once after a router firmware update. mDNS works wired to hub but not from phone on Wi-Fi VLAN.
Fixes
Disable AP/client isolation and guest network for commissioning. Turn off multicast-to-unicast hacks unless documented for Matter. Put phone and hub on same SSID and band. Reboot router; add DHCP reservation for hub. Commission near router; reduce parallel pairing. On prosumer gear, enable mDNS reflector between VLANs or keep IoT flat during setup.
Verify
Hub discovers device within five minutes without Bluetooth-only fallback. Second device pairs without router reboot.