First - this is NOT an Xfinity problem. I had (yes, "had" - we'll get to that) the problem and it was an issue on any wifi connection - home, work (not Xfinity), my brother's house (not Xfinity)... In addition, I have a Samsung S9+ and my wife has an S9. We got them at the same time. Hers has never had this problem. We both use Messages from Google LLC instead of Messages from Samsung. Oddly, we have different versions listed - hers is newer - so I was trying to get that one to load on my phone, with no luck. However, uninstall and reinstall of the application resolved my issue. I had gone through all settings and found no differences, but I may have missed something. I also had the problem with Messages from Samsung and now no longer, so reinstalling the Google app changed something that affected both apps.
For those interested, here are the versions. I'm going to contact Google about that.
5.4.087 Roper_RC15 1/29/2020 mine 5.7.097 Unicorn_RC09 3/25/2020 hers
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