How do I set an .m4r file ringtone I airdropped to my iPhone 13?

I have an old .m4r file on my Macbook, which I used AirDrop to copy to my iPhone 13. There's no way to set it as my ringtone, however. In the ringtone settings page, it's not there, and if I go to the Files app I can open the .m4r file and even play it, but there's no option to "set as ringtone".