All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
There is no doubt that the lights in the cities affect the sight of true dawn as well as the sight of the twilight at ‘Ishaa’. However, we cannot authoritatively assert that it is impossible to see them as the lights vary in strength and weakness from one city to another, and from one area to another within the same city.
What we can say, however, is that if you cannot be sure of the time of the prayer because of light pollution, as you mentioned, or because of the difference of people (regarding the time), or for any other reason, and you cannot find out the time through the Islamic signs, then do not pray until enough time passes with which you become sure or you predominantly think that the time of the prayer has started.
The scholars stated that one should not pray unless one is certain or predominantly think that the time (of the prayer) has started. Ibn Qudaamah said in his book Al-Mughni:
“If a person doubts that the time of the prayer has started, then he should not pray until he becomes certain that the time has started, or he predominantly thinks so….” [End of quote]
With regard to abstaining from eating and drinking for fasting, then you may eat and drink until you become sure, or you predominantly think that the time of true Fajr has started. If you stop eating and drinking before this in order to be on the safe side, then there is no harm. When breaking the fast, you also do not break the fast until you are sure, or you predominantly think that the sun has set.
Allah knows best.