We can assign the ringtone to a group on the Xiaomi smartphone itself, and if a contact in this group is or has been assigned their own ringtone, it will be prioritized.

Android 10 | MIUI 12.0
Step 1:  Open the  Contacts app
Step 2:  Tap on  My groups
Step 3:  Select a  group
Step 4:  Tap on  Ringtone
Step 5:  Choose a  ringtone
  1. Open the Contacts app
  2. Tap on My groups
  3. Select a group
  4. Tap on Ringtone
  5. Choose a ringtone

Xiaomi Instructions