Using one router for two separate networks for Sonos

No clue what that means, no such thing on OpenWrt...

The reason I ask is because you clearly mention a "owner network" and an "AirBnB customer network" ...in OpenWrt, Wireless networks are the same (there's nothing special with a "guest network" excapt it's not connected to LAN1), but I assume the AirBnB are for your guests - a LAN2, correct?

Are you trolling...?

This is why I'm being very clear you need 2 networks, but you keep saying it "cannot be a guest network."

See:

You then add Wireless SSID to the new network - you don't have to name it "guest."