1. Make sure you unlock the DIGITS functionality. (via TMO app or via https://myDIGITS.t-mobile.com)
2. Reboot device afterwards
3. Turn on multi-line settings in the cloud and accounts setting
4. Log in with your T-Mobile ID
In your dialer and native messaging you'll be able to select from which line you want to send/receive from. No need for the app.
Ohhh, The native messenger! Well, that stinks because I had to stop using the native messenger. I switched to Textra because I am unable to send messages over 1MB using the stock messaging app. I liked the stock app but the 1MB limit is a deal breaker for me.
I contacted Samsung and they said it was an issue with T-mobile, T-mobile said it was an issue with Samsung, then Samsung said 1MB was the max. But I guess this rant should be in a different discussion.