Praise be to Allah.
The time when one must refrain from things that break the fast (imsaak) is the breaking of the true dawn, as Allah, may He be exalted, says (interpretation of the meaning):
“and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night), then complete your Sawm (fast) till the nightfall”
Al-Bukhaari (617) narrated from Ibn ‘Umar (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: “Bilaal gives the adhaan at night, so eat and drink until Ibn Umm Maktoom gives the adhaan.” He was a blind man who did not give the adhaan until it was said to him: Dawn has come, dawn has come.
It says in Fataawa al-Lajnah ad-Daa’imah (vol. 1, 10/283):
The basic principle with regard to the fasting person beginning and breaking his fast is the words of Allah, may He be exalted (interpretation of the meaning): “and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night), then complete your Saum (fast) till the nightfall” [al-Baqarah 2:187]. Eating and drinking are permissible until dawn breaks, which is the white thread that Allah has made the end of the time when eating and drinking are permissible. When the second (i.e., true) dawn appears, eating, drinking and other things that break the fast become prohibited, and whoever drinks whilst hearing the adhaan of Fajr, if the adhaan comes after the second dawn, must make up that day’s fast, but if it comes before dawn, then he does not have to make it up. End quote.
Based on that, if a person is in a land where there is night and day, then he must refrain from things that break the fast from the breaking of dawn until sunset, regardless of whether the night is long or short, and regardless of whether the time between dawn and sunrise is long or short. For more information, please see the answers to questions no. 106527 and 2196.
There is nothing wrong with delaying Fajr prayer until fifteen minutes before sunrise, because the time for Fajr prayer lasts from the breaking of dawn until the sun rises, because the Prophet (blessings and peace of Allah be upon him) said: “The time for Fajr is from dawn, so long as the sun has not yet risen.” Narrated by Muslim (612).
And Allah knows best.