Is the prayer of one who combined and shortened prayers for two months, based on a previous fatwa and information from a colleague, valid, or must it be repeated?

The ruling that permits shortening and combining prayers permanently for students studying abroad is a weak fatwa. The correct view is that if a traveler intends to stay for more than four days, he is obligated to pray in full. As for past prayers, they are valid, and there is no need to make them up, as long as you performed them in accordance with the opinion of some scholars.