Saifan, I followed your step-by-step instructions but am still unable to copy over my ringtones, either by drag/drop into iTunes or via the "Automatically Add to iTunes" directory. I tried this on both the OSX and Windows 10 versions of iTunes with my 2015 iPhone 6 running iOS 11. The ringtones I want to add are the free Portal 2 ringtones released by Valve in 2012; these tones were sadly lost from my phone during the upgrade process, which necessitated an iCloud restore for reasons I shan't discuss, and from my iTunes library prompting a download de novo.
This is what my iTunes looks like on OSX when clicking "Edit" next to "Library". There is no option to view or manage tones on my OSX device. Dragging m4r files onto the "Tones" option under device, or into the list of Tones open on the right, have the same effect - which is to say, none, regardless of whether I try to add them all at once or only one at a time.

What other steps can I try to restore these tones?