Why the device failed to connect? (ZigBee)

It is advised to check in the following way:

1) The Wi-Fi of the mobile phone is enabled;

2) The distance between the gateway and the device are moderately distanced, it is advised to be as close as possible;

3) The device may have been added before, it is advised to reset and add again;

4) Check if there are other ZigBee gateways around.