@DoubleB 

hi,

if by main and guest wifi, you are talking about networks (SSID), then you have to go on your "guests" devices, forget the main WiFi and put the new Guest one... i don't see any other solution...

on,y the device can change SSID, the AP can't force the device to go to another network (except if you have control on the devices like in professional environment - but still this will be an action made by the device to switch).