If this is a hanafi masjid and the time of Fajr during Ramadan is way earlier than after Ramadan, you must follow the app as they shouldn’t be trusted with fasting, as they call it way later than when it was due.

In my place of living the fazar prayer time starts at 3:39 (today). But the mosque gives azan at 4:15 or even delay more. But the mosques were very much on beginning time during Ramadan. My question is up to what time I can pray tahajjod and take food to keep fast? Is it azan or actual time from apps?