Here are a couple of things to consider:
- If you are pairing a Thread based Matter device (I think the Innovelli White Series dimmers use Thread instead of WiFI), then you’ll need a Thread Border Router (TBR). A TBR has a wireless mesh side, and a LAN side (and routes between the two sides), and on the LAN side it will send out IPv6 Prefix information and everyone on that LAN that is enabled for IPv6 will pick up this prefix and automatically assign themselves a full IPv6 address. These are ULAs which mean these are private addresses. So if you have a flat (single) LAN based network, then about all that is really needed is to enable HA for IPv6 and that’s about it (one need not enable IPv6 in their router).
- Multicast Discovery. Matter heavily relies on link-local multicasting for discovery of devices when trying to pair/commission a device. Link-local means the multicasting is confined to the LAN it is sent out on. In other words it is not to be routed across from one LAN to the next.
There are techniques/features in certain routers that can be enabled whereby they attempt to relay multicast packets from one VLAN to another VLAN, but these quite often don’t work.