Must I pray alone at the legally prescribed time, or with the congregation that performs the prayer before the legally prescribed time, knowing that the Adhan for Fajr prayer is at the astronomical time without observing the difference in rising points?

If you know the prayer times, then do not pray before the time begins, because the beginning of the prayer time is a condition for its validity. What is written on papers cannot be relied upon if it contradicts the reality. Those responsible for the calendar should take into account time differences or draw attention to them. Otherwise, Muslims must observe them themselves. Advise the mosque congregation gently to delay the prayer until its legal time. If they do not respond, then pray it in your home. However, if you are unaware of the prayer times, and your statement is based on conjecture, then stick with the congregation and do not cause discord.