Game Of Thrones: Double Door

Jason Liu, who accidentally acquired the ability to travel through time and space, can carry supplies to and from the modern world and Game of Thrones worlds. As a result, cheap commodities from modern times have swept Westeros, and a large amount of raw materials have flowed into modern times, which has greatly increased the gdp of the two worlds. By monopolizing the trade between the two worlds, Jason Liu made a lot of money. He was no longer satisfied with being a smuggler between the two worlds, and began to look at Westeros as a battle field Cover not mine this time :( Found it on Mtlnation, https://mtlnation.com/novel/im-going-both-ways-on-game-of-thrones