Display a portrait-mode app on a horizontal rotatable screen

I need to display a portrait-mode app from my Samsung Galaxy S9 on a horizontal PC screen. The PC screen is able to rotate to portrait mode, but the problem is that the image rotates with it. The image is always displayed horizontally in regards to the PC screen. This means that when the PC screen is horizontal, the image is displayed in portrait mode in the middle of the screen, covering only a small "pillar".  When I rotate the PC screen to portrait mode, the image rotates with it, leaving the same "pillar" covered (though now rotated 90°).  What I need is for the image to be displayed in portrait mode while the screen is in portrait mode (meaning the image is rotated -90° when the screen is horizontal, making it utilize the entirety of the screen). I've contacted Samsung, and they say there are no settings on the phone that allows me to do this, so I'm looking for an alternative solution, such as an app or other third-party clients. I've also read somewhere that I have to root my phone (and change some codes?) in order to achieve this, but that seems way too advanced for me. Alternatively, I could use my old Huawei P9, if that helps.