Unfortunately the firmware does not leave a way to manually set the clock (I didn’t think there would be a situation where you have Internet connection but no NTP sync). There are several ways you can set the clock (for example, plug it into another network which permits NTP sync, or upload an Arduino program that manually sets the clock. But in any case, I don’t recommend using the system without NTP, because the time will drift and after several months you may have to do the same trick again.