If the XPRESS switch is in a network that has been deleted or is no longer within range, it must be decoupled. Only then can it be used in a new network.
To do this, go to Devices nearby in the Casambi app and press two buttons on the XPRESS switch at the same time. Now tap on the visible XPRESS switch and then on Decouple. During the time the bar is running down, press any button on the switch. The switch is now decoupled and can be assigned to a new network.