If you want to set up a custom ringtone, please edit in contact view "add a ringtone": Contacts -> Edit contact -> Add -> Add ringtone.

If you downloaded music on your phone and you want to set it as a ringtone, you should add it to the Music folder via PC.

KaiOS devices don't support copy/paste feature, that's why you need to use PC.

Please note that setting up custom ringtones for notifications is not supported.