I think it's just a recent manga I read, but can't find it in my history in my laptop.

Synopsis:
MC is training in a dojo with his grandfather, you can also see his sister there. # of panels later he is transmigrated then he kills the summoner out of instinct since summoner harbors ill intent. The princess (niece of summoner) enters the rooms, MC goes for the kill but gets saved by his servant (which is also a transmigrator). MC spares them then goes on to explores the world and tries to find a way to get back. He also finds a book which details about the adventures of transmigrators and finding ways to get back but all of them says there's no way to get back. (I'm not really sure if the details im remembering are from 1 manga or i have mixed up two different stories lol, but I think I haven't lol)

Apparently his grandfather (got transmigrated before to the same world but manage to get back miraculously), so he trained him all his years in preparation for if ever his grandson gets transmigrated to that world.

In the real world also investigators go to their house to investigate about his grandsons disappearance, then when the little sister escorts out the investigators, his grandfather in another room senses that her granddaughters presence is gone and his 2 swords is also reacting to it and he feels that it's the same feeling when he was transmigrated, and he runs and also goes in to the magic circle/portal/summoning in hopes to save his helpless granddaughter = last chapter i read ithink.

I think it's a recent one, i prolly read it on my ipad but it's in private mode so no history to backtrack haha

EDIT: nvm found it https://www.mangaupdates.com/series.html?id=140719