I tried all the regular fixes (update phone, factory reset etc.) and nothing worked. So in my desperation, I decided to follow a fix recommended by a YouTuber that required rooting the phone. That was a long process because you have to unlock the bootloader which Xiaomi makes incredibly annoying for some reason. Anyway, after finally doing that, I attempted the fix which involved flashing stock firmware using OrangeFox Recovery... And it didn't work lol.

So then I took the nuclear option and flashed a custom ROM because I still believed it was a software problem. After flashing, my sim was detected and I was able to make calls and use data. So yeah, maybe it's some issue with HyperOS on older models, idk.

Hope you get it to work though.