The red light means the battery level of sensor was low. Please place sensor under sunlight for a couple of days to charge. If sensor blinks a few second white light followed by a red light and immediate shut down, then the sensor is charged but it failed to connect to your home WiFi. Please press the button 6 times quickly( turn it on and off 3 times ) and the light will start to blink slowly. Please take a look at this video:
If you are able to make the light blink slowly, you are ready to reconnect it to your home WiFi. Please go to the Netro app -> settings-> sensors -> select the sensor -> Change WiFi network/password to reconnect it.