maybe you miss this :
Real Time Clock (RTC) Related
INT (Jumper type: Solder bridge, Location: Bottom, Factory state: NOT connected by a thin track)
Close this jumper to allow the Real Time Clock (RTC) to wake the ATmega328P at a specified time. This jumper will connect INT of the RTC Chip (DS3231) to PD2 (INT0) of ATmega328P.
regards,
deray