I looked at 2 novel websites, the NovelUpdates and MLTREADER,
https://www.novelupdates.com/series/isekai-tensei-no-boukensha/? and
https://www.mtlreader.com/novels/90. (There's going to be a bunch of links) So let's start off with
https://europaisacoolmoon.wordpress.com/2021/02/09/volume-2-chapter-17-conclusion/ < This, this chapter ends off with
“It’s the sword I used to use, I tried to make it look Japanese. I hope you’ll use that weapon from now on.”
“I gratefully accept. I’ll come back to play again sometime. [Transfer]”
Cain disappeared in an instant.
“What a lively fellow. But still, to have grown to that height… maybe Doran went a bit too far.”
While saying so, Yuuya poured himself a second cup of coffee.
With the next chapter starting off with
https://europaisacoolmoon.wordpress.com/2021/02/10/volume-2-chapter-18-summoning-magic/
"Today was a school day.
Recently, Cain had been absent a lot due to various reasons, but now he was finally able to go to school normally."
As you know if you've read through all 30 chapters, there was a point in where Cain tells the King about his 5 year adventure (Chapter 27 of this manga) which there is none in the novel. But if you go to this novel
https://www.mtlreader.com/novels/90/chapters/18481 < This tells us about chapter 27 of the manga saying
"Cain, this is the seal of the Royal House of Esfort. I stamped this on
He removed the markings and began to read inside.
His Majesty's complexion gradually changed as he began to read
After reading everything, I handed the letter to the Prime Minister and began to read it, but his complexion changed as well.
I'm not going to be able to get rid of it. Cain, my lord, you were with the first generation and that you have become a god.
Yes, that's right. The First Lord created another world and was the new Creator God of that world."
with the chapter ending with Cain telling about his 5 year adventure, but in the fucking next chapter
https://www.mtlreader.com/novels/90/chapters/18482 < It doesn't explain about the summoning but instead starts with
"Since the previous day's chastisement was successfully over and the school was on holiday, Cain showed up at the Adventurer's Guild for the first time in a long time.
Since it was before noon, there were few adventurers and the reception area was also deserted.
The receptionist, Letia-san, was there, so he called out to her."
What I'm trying to say is that the novel is unorganized or there is 2 different versions of the novel.