Samsung is the one that controls the software expience for your device. If the functionality is gone completely than that was a design decision on Samsung. It may just be placed somewhere differently though. I don't have any expience yet with the S9 since it is so new and I never used this feature. However, if you contact T-mobile someone should be able to walk you through the steps if the feature is there. Contact Us
I wanted to add that you can also work with Samsung directly as this is there device and they should be able to assist you as well.
So I was seeing throughout the forum and what came up is that users are using a 3rd party app called textra to achieve the custom tones and it seems that the feature is completely gone for text alerts. You can still customize call ringtones however.