Is what a preacher said true, that there will be a wall of fire on the Day of Judgment, from which a new messenger will emerge for the people of the Fatra (those who lived in periods without a prophet) ordering them to enter the Fire, and that this messenger belongs to a different world, knowing that Prophet Muhammad is the Seal of the Prophets for this world?

One must believe that Muhammad, peace and blessings be upon him, is the Seal of the Prophets and Messengers, and that there is no prophet or messenger after him. Definitive texts from the Qur'an and Sunnah have been revealed to that effect.

As for the People of the Fatra (those whom the message did not reach), the preponderant opinion is that they will be tested on the Day of Judgment, where Allah will command them to enter the Fire. Whoever obeys will enter Paradise, and whoever disobeys will enter the Fire. This command will come directly from Allah or through an angel, and not through the sending of a new human messenger.