The only option on MIUI is to root the phone and follow this tutorial:
A lot of people have complained about stuttering in between App switching and gaming. This guide helps you disable Dynamic refresh rate through MagiskHide Prop Config, which is easier than setting it manually and safer to disable if it doesn't...
forum.xda-developers.com
It disables dynamic refresh rate, so when you select 120hz it'll be 120 all the time, and when you select 60hz it'll be 60 all the time.
This fixes frame drop in games like CoD Mobile.
If you want to go to a custom ROM, many of them have an option to choose between 60, 90 and 120 hz globally and per app.