Moto Tansakusha no Ojiichan: Mago ni Segamarete Dungeon Haishin wo Hajimetan ja ga, Nazeka Bazuriotta wai

Many years have passed since "dungeons" began to exist in the world. "Grandpa, stream a dungeon run for us! You used to be an explorer, right?" Urged by his grandchild, former dungeon explorer Genji Kudō decides to dive into a dungeon for the first time in 30 years. But Genji's dungeon strategies, shaped by the old ways rather than modern conventions, start to pile up unexpected results—until his adventures go viral and cause a sensation across the nation...!?

(Source: Mag Garden, translated)