Unfortunately it seems that this approach copying mp3 files into free accessable folders called R|ringtones, N|notification, sometimes under media, some times under media/audio etc. does no longer work with Android Pie. At least I have the OnePus 6T with Android Pie by default, and whatever I am trying is not working. Before with my OnePlus 3 and all the other Samsung phones before this worked like a charm! Google, what have you now done again?
Edit:
But there is for example an app called "Tone Selector (Ringtones/Alarm)" which exactly can do that. It allows you to specifiy the path, e.g. where you previously copied the files (Ringtones, Notification, etc.) and then you save this, and afterwards it is selected. It even works for SMS, when you confgured that SMS shall used the default notification tone.