Some customers who have linked alarms in a series, and then later to run them on APP, find that the alarms do not sign in to APP. If you experience the same problem, this may be the solution: Set an alarm in Learn mode and let it connect to itself, let it flash for 15 seconds, put it back in normal mode and disassemble the battery. This will reset the alarm's series connection to the other alarms. One for one repeated procedures with the other alarms.