I have solved the problem of all my missing ringtones after upgrading from an iPhone 6S Plus running iOS 11 to my iPhone 8 Plus running iOS 11.
I have a PC so these instructions work on a PC. I do not know if they work on a Mac.
Connect your iPhone with the missing ringtones to your PC that is running iTunes 12.7
After it connects, you will see your iPhone listed in the left menu, click on it to open it up. Scroll down to where you see TONES and select TONES. A window will open up and it will most likely be blank.
Now, in the upper left corner, click on FILE and then click on ADD FILE TO LIBRARY.
This is where the hiccup is with iTunes. In the Windows window that opened, find your ringtone, click on it once and it will be shown as the selected file. Then...do not click on Open yet. Select the file again and this time, drag it to the TONES area and drop it. Nothing will happen. Go back to the windows window and click OPEN. The file will now appear in TONES.
After you have selected and added all the missing ringtones, click on SUMMARY, then SYNC the phone. Wha la, your ringtones are back and you can now set all the tones up.
This also will work to restore all your missing PDF's