What do you mean by installing it to the iPhone? Because it obviously does that when you follow this path
Go to the list of projects --> triangle icon in top left --> My songs --> Your new song --> Tap and hold it --> Pop up Menu --> Share --> Select Ringtone --> Export --> use sounds as --> select the proper option.
Standard Ringtone: regular ringtone.
Standard Text Tone: text messages.
Assign to Contact: particular contact in your address book.
But I don't know about iOS 13. This is the usual I do in iOS 11.