Manage connectivity of apps to internet of Sim-card vs. wifi
Turn off any power saving mode or data saving mode. If u have limited the data per month or per day, then turn that off.
Run an Internet Speedtest from the sim card data. If you are having trouble run Speedtest may be there's an issue, so try that sim on another phone and check.
I think MIUI has an option for turning on/off mobile data used by app. You can find it in App Management (from settings) for a particular app.