Maybe I didn’t understand the question, but when there is no Internet, you can’t see the equipment that is connected via the Zigbee channel. I also noticed this feature: if the equipment that has a Zigbee channel (in my case, the T1 lamp) is disconnected from the power supply (100% broken Zigbee channel), it still continues to show in the application for some time that everything is OK, the lamp is not “broken”.
Also note that when the Internet is restored (possibly due to a failure of the industrial network in the house), in order for the gateway to see your device, you need to send a command to it (the gateway) from the device (I’m here), I don’t know the period of sending a periodic communication signal.
In a temperature sensor, for example, such a signal passes once an hour (temperature measurements) or at the moment of a temperature change of 0.5 °C.
For general development here, I did some research on the Zigbee channel, you can read Water leak sensor dropping off - #11 by gafich10