Rejoice the updated better iTunes with less features! (A big sarcasm)
iTunes 12.7 removes the ability to manage and sync ringtones, especially custom ones. This is just one of those big things that stops me from updating iTunes. Your best bet is to find a third party app that allows you to sync tones to your iPhone ex.
iExplorer and iMazing are two applications that might suits your needs. But neither are cheap. Be sure to try them out before paying the price.