120Hz, and 120FPS are two entirely different things, one does not mean the other, and vice versa, try reading this article, as it explains, better than I can, the difference:
Also if CODM does not list 120FPS as an option, it would be on their end, as they did not program this to work on your device, NOT Samsung's fault, as they have no part in the development of third party games.
Stealthy gaming is a web site. They are suggesting you load an app called VMOS, and then follow the step by step tutorial that is on the site I linked. I found this all by googling "how to get 120fps in Call Of Duty Mobile", I personally know nothing of the site, or the app.