To check whether Mi Door and Window Sensor 2 can't connect to Home Hub, please refer to the following steps:
1. Press the reset button of the Door and Window Sensor 2. If you hear voice prompts like "Normal link confirmed" from the Hub, it means the sensor is communicating well;
2. If not, move the sensor closer to the Hub and try again. If you still don't hear prompts, the sensor might need re-adding or battery replacement;
3. Move the sensor unit close to the magnet unit and separate them 2 or 3 times;
4. In the Mi Home app, go to the Door and Window Sensor's 2 home page and refresh the device records (log). If you see related logs, it means the device is working correctly;
5. If the issue persists, delete device in the Mi Home app and add again.