I got a message that the smoke sensor is offline but it has battery.

If the alarm is offline but has battery, it may mean that it has lost contact with the system. Note that it will still report fire locally, just like a normal smoke detector. If this happens, you must contact support so that we can restore the connection. It is also important that you are at home when you call. The number for support is 23506847 and is available between 08-16 on weekdays.