We can also specify that apps on the Xiaomi smartphone can only access the mobile data (or WLAN), and if we allow an app to connect to the mobile data only, the mobile network will be used even if the WLAN connection is active.

Note: If we use dual SIM on the device, we can set the setting for each SIM card individually.

Android 10 (MIUI 12.0)
Step 1:  Open the  Settings
Step 2:  Tap on  Connection & sharing
Step 3:  Tap on  Data usage
Step 4:  Choose an  app
Step 5:  Enable or disable  Mobile data-SIM 1 / 2  or  Wi-Fi
  1. Open the Settings
  2. Tap on Connection & sharing
  3. Tap on Data usage
  4. Choose an app
  5. Enable or disable Mobile data-SIM 1 / 2 or Wi-Fi

Xiaomi Instructions