I have recently bought a Sonoff Zigbee bridge and door sensor.
The installation of the bridge and pairing of the door sensor worked perfectly.
Since then, the door sensor repeatedly shows as online for 50 minutes and then offline for 70 minutes. It follows the same pattern all the time now.
The bridge has excellent 2.4Ghz wifi signal and remains online in the app all the time so I know that the WiFi part is ok. When the door sensor is offline, the bridge is always online so the suggestion in the app to check the internet connection can;t be at fault. It is a 1Gbit fibre connection that is always up. All of the Sonoff hardware is 4 days old and the battery is 100% full.
The door sensor is less than 3m from the bridge and there is nothing except air in between them so the Zigbee signal must be fine as well.
The issue is that I cannot trust if the door is open or closed when the sensor is offline. This makes the product unusable in its primary purpose.