Sonos and Chromecast wGTV connecting to 2.4g

Ya something that IoT mfrs fail to realize that 2.4 and 5Ghz are on same network but phones and pads prefer 5Ghz if detected. Something there setup SW doesn't support which is a failure on there part. 

Unfortunately the 350 series doesn't have later FW and doesn't seem to support the separate IoT network has more higher model Orbi systems do. 

If you tried reducing the 5Ghz power on the RBR with all RBS turned OFF and you cant get your devices to fully connect to 2.4Ghz long enough to setup the IoT device, something you can try, find you a 2.4Ghz only router, use it and configure it for same SSID name and PW as what you have on the Orbi system. Connect your phone or pad and the IoT to this router, once you have it set up on this separate router, turn it OFF. The IoT device should auto migrate and connect to the Orbi system. 

Or find you a separate 2.4Ghz wireless AP that you can connect and use a separate wifi SSID name and wifi channels away from the Orbi channels and use that as a 2ndary AP. 

Or get you into a more higher Orbi model that has the separate IoT network that can be configured for 2.4Ghz only needs.