We would suggest following:
*Re-creating a pair and refreshing all SONOS connection (Outputs > Sonos app (Client) ).
*Reach all outputs and paired outputs using SONOS native app on your iOS device.
*Start VOX and check output list to see all outputs available.
Also, you may try to turn your internet off, but WIFI should still be working, to check if you see a SONOS speaker with internet disabled.