Whenever I one tab in desktop site (on the same site), the other tab also changes

For context, I am a mobile user. Recently, one of those white box pop-up-ish things showed up, saying that whatever site I put desktop, would always says that unless I turned it off. So, basically, if I wanna view 2 tab of the same exact site, one in desktop mode, the other in mobile mode, I’m out of luck. I’ve been trying to find a way to turn it off, but not only can’t I find whatever feature keeps it ons, but I tried to find solutions myself… and almost bricked my phone. If anyone know what I’m talking, please help, thanks in advance.