How to get Sonoff SNZB-02D Zigbee sensor back online

I have a Sonoff SNZB-02D Zigbee temperature / humidity sensor that has gone offline, I think because nearby Zigbee routers had been turned off for a couple of days. Those routers are back on now but the SNZB-02D hasn’t returned. The sensor is working and displaying the temperature/ humidity on its LCD screen as expected.

What should I do in this scenario?

Thanks in anticipation

I suggest turning the device off and then on again. If battery operated, remove temporarily the battery.

Put Zigbee in pairing mode and hold the button on the back of the device for 10 seconds.

Thank you, this worked. I considered doing this but it felt like a sledgehammer to crack a nut!

Why didn’t the battery powered Zigbee sensor rejoin the mesh once the mesh re-expanded i.e. when the nearby router turned back on?