Just thought I'd pass along a little useful information regarding the T-Mobile app when it will not connect or you cannot even get to the login screen.
I myself had had no problems for a great long while and then all of a sudden one day I couldn't log in I couldn't get to the login page and kept getting the error to try again later. No matter how you see that error this is how I fixed that problem.
It seems as if it had begun soon after the last update to the app. After uninstalling and reinstalling I decided to check the app permissions. I discovered that the app had no permissions set at all and during the setup of the app I was never asked to allow any permissions.
I believe this is what the problem was. So I went into my Android and went to settings and then to apps and then found the T-Mobile app and tapped on it and tapped on app permissions and enabled permissions.
This seems to have fixed the problem however I'm not sure why the app never asked for permissions to begin with. Perhaps someone will find this helpful.
I'm using a Motorola Moto X4 with Android 9 however I do not believe that this error is related to Android 9 or an unlocked device. I believe it was just a missing step in the app set up after the update.