I Got a Cheat Skill in Another World and Became Unrivaled in The Real World, Too

The story

All his life, Yuuya has been bullied at school and neglected by his parents. After moving into his late grandfather’s home, he discovers a strange door that seems to be calling out to him. Yuuya opens the door, steps inside, and is suddenly transported to a magical world! This new world holds rare treasures and grants him powerful skills, which he takes back to Earth for a new lease on life. (Source: Crunchyroll)