The process to unsync a Zigbee converter from gateway is the same as a smart clamp.
Power reset the device, then within first 5 seconds on boot up press and hold the synch button for 3-5secs.
The pair lit will then be lit solid red.
Once the device is back solid red the customer can sync the device via the normal process.
Be sure to leave the Gateway turned off or the device may automatically resync again.