If brand new, when powered on it will attempt to pair to Zigbee2mqtt automatically. If not (or if has been paired before and needs to be re-paired) - press and hold the (relay) button on the top for about 5 seconds until the relay clicks and the red LED flashes several times. The device will then go into pairing mode and the blue LED will begin to flash. When connected, the blue LED will turn on solid. It should then be connected to Zigbee2mqtt. Pressing the button should activate the relay on/off as normal, and the red LED will be on/off.
It does not contain an esp, so impossible to flash, but that is not needed for zigbee. For the rest, just as the Wifi variant it is an on/off switch you can control from HA. It does work as a ZigBee router to create a stronger Zigbee mesh.