Is a child born as a result of a marriage contract agreed to be invalid, where the couple continued to cohabit after the divorce, attributed to the father, especially after the mother abandoned him?

You must increase your good deeds, strive for righteous companionship, and avoid bad company. If the marriage is considered valid, the child is attributed to you. The child born to the woman four months after your separation from her is attributed to you, because if she gives birth before the maximum period of pregnancy has passed, the child is attributed to her husband. This is clarified in "Qawa'id al-Ahkam" by Al-Izz ibn Abd al-Salam.