Hello mmmeline,
Welcome to Apple Support Communities.
We understand that you want to sync your custom ringtones to your iPhone. We are happy to help.
Depending on how you created the ringtone, the steps in this link might help you: Create a ringtone of your Logic Pro or GarageBand for Mac song - Apple Support
"To create a ringtone on your Mac, prepare it on your Mac using GarageBand or Logic Pro, then share it to iCloud Drive so you can open it in GarageBand on your iPhone or iPad. Once it's on your iPhone or iPad, export the song as a ringtone from GarageBand, then assign the ringtone. Or just create the ringtone directly on your iPhone or iPad.
Set up iCloud Drive
To share songs from your Mac to GarageBand for iOS, you need iCloud Drive. If you haven’t already, set up iCloud Drive on your Mac and your iPhone or iPad.
Share your project
- On your Mac, open the project in Logic Pro or GarageBand.
- Share it to GarageBand for iOS:
- In GarageBand, choose Share > Project to GarageBand for iOS.
- In Logic Pro, choose File > Share > Project to GarageBand for iOS.
- Select the "GarageBand for iOS" folder in iCloud Drive, then click Save."
Let us know if you have further questions.
Take care.