Responses
I am not really sure because i didn't read the whole thing but maybe it's The beginning after the end.
http://www.mangago.zone/read-manga/the_beginning_after_the_end/
unfortunately it wasn't this one but thank you!
I don't remember much but it was a typical op reincarnation manga and they found out that the ml wasn't from the world 1, because he was a tiny kid and 2, because he could read an ancient book made by a previous king(?) for people that came from japan. I think he also made the king a chessboard?