Did you read the tip?
The section Add ringtone to device, iTunes 12.7 or later explains how .m4r ringtone files are added to your device with the current version of iTunes. You don't have to adjust any of the iTunes menus. Connect the device to iTunes, drag and drop from Windows Explorer to device where it appears in the sidebar.
The section Removing hidden tones has some troubleshooting advice if iTunes isn't able to add the tones using the method described.
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