Add personal ringtones

Use GarageBand.

Yes, if you have GarageBand, Transfer the songs from the Files app to GarageBand then --> GarageBand: Share songs to iTunes - Apple Support or Import a song from the Music app into GarageBand for iPhone ...

Here you have the process!

My GarageBand opens my iCloud Drive. In your case, it may not be so. Nevertheless, the process remains the same. This helps because accidentally if you open up iCloud Drive using GarageBand you can get back to GarageBand.

Music must be on your iPhone not on iCloud Library.

When you select the song Hold and tap the Song delicately for it to populate it in Audio Recorder.

Watch the GIF below to convert any music to RingTone

This is made on iOS 13.3.1 iPhone SE

Here is the link on changing your ringtones but if you want a ringtone from a song you like you will have to pay for the song you want as your ringtone on iTunes. If you already made one or don’t want to pay for a song you can make yourself a custom ringtone but will require a Mac with Finder or Windows with iTunes. - https://support.apple.com/en-us/HT207955