Toaru Majutsu no Index: New Testament - Volume 18, Chapter 4: A Worthwhile Battle at Heaven’s Peak - "Light".

Doomsday comes, and human beings enter the heavenly world.
Ten years later, Yang Chen, who was betrayed tragically, returned strongly with the Tianyu bug. In this life, he vowed to make the betrayal pay the price. He has a powerful bug bonus, just like an open hang. Frightened to hear…

- Description from novelbuddy

Sixty Years ago, one middle aged man brought a Qilin that looked like a pug and a mandarin duck that looked exactly like a duck, walking into the Imperial City of the Central Plains for the first time.

Long Fei, a cross-timer and doujin, is an otaku who traveled to another world. In the original world, he was an otaku fan of Gundam and military science fiction.

He died in an accident and was reincarnated into another world. The Gundam works he had read before his death and the techniques in various military novels could be brought to another world.

How would he use the Gundam that Long Fei created? What waves can be caused in this different world?

“When I left you, it wasn’t because I was materialistic.”

“I know.”

“He doesn’t love me. I already broke up with him.”

“I know.”

“You knew? I didn’t know you were still so concerned about me…”

“I know because he and I are now a couple.”

This is a story about how a poor loser turned into a tall, handsome and rich guy and snatched his ex-girlfriend’s current boyfriend away.

- Description from Novelupdates