Ringtones are to be selected easily in Contacts app as previously explained and not in Settings !
Sorry but you are doing something wrong ... On my OnePlus Nord all my contacts have different ringtones since 4 months. And Google Communication Suite is far more sophisticated that you could dream !
If you do not know how to do this, go here :
OnePlus global support (call, email, chat) :
https://www.oneplus.com/global/support/contact
All countries selectable.
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