So, when I go back to settings > sound & vibrations > ringtones and press the + icon, it says No Folders. I have no idea what seems to be the problem to why I can't get it as my ringtone. Everybody else seems to have no fault in getting a custom ringtone.
I don't know if it could be your case, but if your phone is connected to your computer with UMS (USB Mass Storage) enabled along having a custom ringtone stored on your SD card (/ringtones folder) instead of MTP this will give you this kind of problem.
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