The Master of Ragnarok & Blesser of Einherjar

Synopsis

Some urban legends are best left untested! Yuuto Suoh gets more than he bargained for when he joins his childhood friend Mitsuki Shimoya in testing out an urban legend. When he uses his phone to take a picture of himself with the local shrine's divine mirror, he is whisked off into another world – one heavily steeped in the lore of the old Norse myths. Using his knowledge gained from school and from his solar-powered smartphone, he has the chance to bring the Wolf Clan, the same people who cared for him, to prominence, all while earning the adoration of a group of magic-wielding warrior maidens known as the Einherjar. (Source: J-Novel)

Synopsis via AniList.