Sunnah prayer for one who misses Fajr prayer in congregation
But if he delays it until after the prayer or after the sun has risen, there is nothing wrong with that. But it is preferable to start with it, as the Prophet (blessings and peace of Allah be upon him) did when he missed the prayer on one of his journeys.
If he comes to the congregation and finds them already praying and he prays with them without having offered the Sunnah prayer first, he may offer it after the (obligatory) prayer, to make it up, or he may offer it after the sun has risen and become high, which is preferable. Both options are mentioned in the hadeeth, and he has the choice, in sha Allah: he may offer it after Fajr, in the mosque or at home, or if he wishes he may offer it after the sun has risen and become high. The matter is broad in scope, praise be to Allah, and both are mentioned in hadeeths from the Prophet (blessings and peace of Allah be upon him). End quote.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him)