Ahh, it's this novel.
Short summary: Guy gets the ability to travel between his world and a post-apocalyptic Fallout-esque world. Story revolves around him taking goods from his world (food, etc.) and exchanging it in the future fallout world for stuff like genetic enhancements, gold, building a harem, futuristic tech, building a fortification etc. So everybody in his original world goes "wow what an amazing superhuman and genius who has all these amazing inventions and tons of money" while the Fallout world people goes "holy shit, look at how much food he has and how generous he is giving food in exchange for so little".
Decent premise but the dialogue was dry and the MC & his harem doesn't have a single brain between them all (MC: I want go fug pretty girls. Harem: Oh man, I am instantaneously and suddenly in love with the MC for little to no reason). Apparently, a decent amount of racism as well. If you like the idea of the "buy low, sell high" idea or a harem horde and are fine with the negative points I mentioned, then it it may be worth a read. Hopefully, the manhwa does a better job.
Original novel: https://www.novelupdates.com/series/i-have-a-mansion-in-the-post-apocalyptic-world/