I was away from my desk over the weekend. I just got home and tried it myself. On my Mac (Mini M4 running macOS 15.7.4) and iPhone (13 mini, iOS 26.4), I did it this way:
- Connect the phone to the Mac via a USB cable
- Open Music (1.5.6.11 on my mac)
- Select the phone in the Music sidebar
- Drag/drop the M4R file to the phone’s window (which only shows the music tracks sync’ed to the phone).
On the phone, i saw the new ringtone on the list available via Settings → Sounds & Haptics → Ringtone (and the other categories on that page, like Text Tone, New Voicemail, etc.)
Of course, it it’s a sound you want for an alert like texts or reminders, you probably want something very short - no longer than 2 seconds. A 30 second music clip whenever you receive a text would get annoying really quickly.
The track never appeared in Files. (If you want to delete a ringtone, you can swipe-left from the list of ringtones on any Settings page where you are able to select one.)
Update
I also did it from the Finder. I selected the phone in the sidebar, then picked the Music tab (where you can configure music sync), and dragged the M4R to that screen. It also added the sound to the set of ringtones.