The Levi’s were considered priests in Egypt, who appointed them?

The Ramban (Shemot 5, 4) states that the tribe of Levi were exempt by the word of Pharaoh from the hard labor of the enslavement as they were the teachers and the wise men of the Jewish people and as such did not share the hardship like their brothers, it does not seem to indicate they were Priests in the simple meaning as the Torah was not yet given and the Korbanot not yet offered in an obligatory way.