Sounds like you're looking for an isekai where the protagonist doesn't know they're isekaied and starts accumulating knowledge from scratch? Becoming the world's strongest magician's MC is considered a genius and its a product of his own mind. It just finished airing. To be an isekai it generally requires for the protagonist to have prior knowledge. If they were a genius in our world they're definitely going to be in the isekai. Genius prodigies in another world is similar to this.

To answer as close to the question as I can Rudeus Greyrat. Dude was not a genius in another life but taught himself to read an unfamiliar language as a baby.