I am having the same problem, I can connect to the app on WI-FI but not on data. I have Metro PCS (on T-Mobile network). Like another user pointed out, this issue started with the latest patch.
HOWEVER, my wife is on my Metro PCS plan and she has absolutely no problems logging into the app on either WI-FI or data on her phone. She has an older Samsung J7, I have a Samsung S9.