Praise be to Allah.
If the fasting person is not sure whether dawn has broken, and he has no clear indication that it has definitely broken, if he has sahoor at this time, there is no blame on him and his fast is valid.
That is because Allah, may He be exalted, says (interpretation of the meaning):
“And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]. Then complete the fast until the sunset”
So stopping eating and drinking is connected to certainty that dawn has broken, not to merely thinking that that may or may not be the case.
Shaykh ‘Abd ar-Rahmaan as-Sa‘di (may Allah have mercy on him) said:
“And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night].” This refers to the time when one should stop eating, drinking and having intercourse.
It indicates that if a person is eating and so on, and is not sure whether dawn has come, there is no blame on him.
End quote from Tafseer as-Sa‘di (p. 87).
Ibn Qudaamah (may Allah have mercy on him) said:
If someone who is not sure whether dawn has broken eats, and the matter is not clear to him, then he does not have to make up the fast, and he may continue to eat until he becomes certain that dawn has broken. This was stated by Ahmad, and is also the view of Ibn ‘Abbaas, ‘Ataa’, al-Awzaa‘i, ash-Shaafa‘i, and as-haab ar-ra’y. Something similar was narrated from Abu Bakr as-Siddeeq and Ibn ‘Umar (may Allah be pleased with him).
That is supported by the verse (interpretation of the meaning): “And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]”, so the time for eating extends until one becomes certain, and that may be preceded by some uncertainty. If it were obligatory for him to make up that day, then it would be forbidden for him to eat [at the time of uncertainty].
The Prophet (blessings and peace of Allah be upon him) said: “So eat and drink, until Ibn Umm Maktoom gives the adhaan.” He was a blind man, and he did not give the adhaan until he was told that dawn had broken.
So the basic principle is that it is still night, and the time of uncertainty is part of the night, so long as it is not known for certain that night has ended.
End quote from al-Mughni (4/390-391).
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
If he is not certain as to whether dawn has broken or not, he may carry on eating and drinking until he is certain that dawn has broken, and if he comes to know after that that he ate after dawn had broken, there is a difference of scholarly opinion as to whether he is obliged to make up that day.
The view that is more likely to be correct is that he does not have to make it up. This is narrated from ‘Umar, and was also the view of a number of the earlier and later generations. And the view that he must make it up is the well-known view among the fuqaha’ of the four madhhabs. And Allah knows best.
End quote from Majmoo‘ al-Fataawa (25/216-217).
With regard to the time that you asked about, if in your city that is a time when there could be uncertainty as to whether dawn has broken, then there is nothing wrong with eating and drinking at that time, for the one who intends to fast, and his fast is valid.
And Allah knows best.