If you're having too much trouble connecting the phone to the PC via USB, you could also make use of your Google Drive in the cloud:
1. On your computer, go to drive.google.com in your browser of choice, and make sure you're signed into your Google account there.
2. Also open your file manager on the computer as a smaller window (i.e, don't make the file manager full screen), locate the ringtone file, and drag and drop it into the main Google Drive screen -- this should upload the file (most likely to your main My Drive folder in Google Drive).
3. Now, on the phone, open your Google Drive app, make sure My Drive is highlighted at the top, and scroll down. You should see the file there. Click the 3-dot button associated with the file, and then choose Download.
4. The file should now be in the phone's Download directory, and therefore available to be used by the system as a ringtone.