I forget the title of the novel but I recall that it was a Chinese novel about the mc being able to travel to other world. That world was a couple 100(?) years after the apocalyptic that world have higher technology but lack food. It use energy as one of the form of currency. The first chapter the mc was captured by a female who he tried to save from hunger. After that he decidedly not hired the woman as his bodyguard. Please, someone help. I cannot remember the title of this novel and I really am at the end of my rope.
I really am not sure since I only read the synopsis but according to your title, it may be i have a mansion in another world (https://www.webnovel.com/book/7143532406000605)
The correct answer has already been posted, but another story with an MC that can go to and fro between worlds: Jikuu Mahou de Isekai to Chikyuu wo Ittarikitari.
That's... partially right and wrong I guess. It's not like he didn't hire her for bodyguard duties, but he also 'tried out' her extremely close-quarters skills, so that she can protect him during the nights as well. So I guess she's a Bodyguard/Girlfriend/Concubine/His woman now.