Will someone who missed the Fajr prayer in the mosque and prayed it in congregation at home before sunrise attain the reward of praying in the mosque and being under Allah's protection? And is he sinful for not praying it in the mosque?

If it was a person's habit to pray Fajr in congregation and they were late for an excuse, such as sleep or illness, there is no sin upon them, and it is hoped that they will attain the reward of one who prayed in the mosque. The full reward of the congregation will be written for them, as if they were present, based on the Prophet's (peace be upon him) saying: "If a servant falls ill or travels, there is written for him the like of what he used to do when he was a resident and healthy." Furthermore, whoever prays the Fajr prayer in congregation is under the protection of Allah.