Praise be to Allah.
The basic principle is that each prayer should be offered at the time appointed for it by sharee’ah, because Allaah says (interpretation of the meaning):
“Verily, As‑Salaah (the prayer) is enjoined on the believers at fixed hours” [al-Nisa’ 4:103]
Joining two prayers is not permissible unless there is a reason such as travelling, rain or hardship. If the time for ‘Isha’ is very late and the time for Fajr is early, such that it will cause hardship to offer ‘Isha’ on time, in that case there is nothing wrong with joining it to Maghrib at the time of the earlier prayer.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked about a country in which the disappearance of the red afterglow which signals the onset of the time for ‘Isha’ comes very late and it is too hard for them to wait for it.
If the red afterglow does not disappear until dawn comes, or it disappears at a time when there is not enough time to pray ‘Isha’ before dawn comes, then they come under the rulings of those who have no time for ‘Isha’. They should estimate its time based on the nearest country to them where there is considerable time to pray ‘Isha’, and it was also suggested that they should base it on the prayer times in Makkah, because it is the mother of cities (Umm al-Qura).
If the red afterglow disappears a long time before ‘Isha’ and there is enough time to pray ‘Isha’, then they must wait until it disappears, unless waiting for it will cause them hardship. In that case it is permissible for them to join ‘Isha’ with Maghrib at the time of the earlier prayer, so as to ward off hardship and difficulty, because Allaah says (interpretation of the meaning):
“Allaah intends for you ease, and He does not want to make things difficult for you” [al-Baqarah 2:185]
“and [Allaah] has not laid upon you in religion any hardship”
In Saheeh Muslim it is narrated from ‘Abd-Allaah ibn ‘Abbaas (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) joined Zuhr and ‘Asr, and Maghrib and ‘Isha’, in Madeenah at a time when there was no fear and no rain. They said: What did he intend by that? He said: “He intended that his ummah should not be put in difficulty” i.e., that they should not be faced with hardship due to not joining prayers. May Allaah help us all to do that which is good and right. End quote from Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen (12/206).
And Allaah knows best.