I could always sent PXT's on my wifi to save cellular data. Now I must turn my wifi off and use only cell phone data to send. I am with Xfinity Mobil. Contacted them twice they said it is in my phone. Called Samsung (Galaxy S9+).
Samsung ran some test and said everything on phone is not causing it, they recommended me to go to Best Buy/Geek Squad and have phone analyzed. Geek Squad said phone in great shape, upto date on hardware and software and running as expected.
They added a note: that this is a common problem with Xfinity Mobile, could be a bug, could be intentional.