Matter discovery uses mDNS multicast on the local segment. Separate VLANs without reflection strand phones on VLAN 10 and hubs on VLAN 20 — accessories never appear.
Symptoms
Works when everything on default LAN; fails after IoT VLAN project. UniFi/Omada users see devices until VLAN rules applied. Home Assistant Matter on different subnet than Wi-Fi AP.
Fixes
Commission with phone, hub, and device on same VLAN temporarily. Enable mDNS reflector / Bonjour gateway between IoT and main LAN per router docs. Do not block UDP 5353 between segments. For HA, run Matter on host NIC that bridges Wi-Fi clients or configure documented reflector.
Verify
Hub discovers new device within five minutes on intended VLAN layout.