Question: how to set purchased song as ringtone on iphone without computer?
Answer: Tap the Share icon, followed by the Ringtone icon. Name the ringtone, hit export. When finished exporting tap Standard Ringtone to set it as the default ringtone of the device. Now go to iPhone Settings app -> Sounds -> Ringtone and here you will see the newly create ringtone as the default ringtone of the iPhone.