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 (Xfinity), work (Verizon), my brother's house (Cox)... 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, although I tried both and had the problem with both. Oddly, we have different versions of the Google app 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 Google Messages application resolved my issue. I'm not sure what setting I may have missed. I thought I had checked them all - even the advanced, and I tried the metered suggestion listed here which didn't work for me. So no smoking gun... just a possible solution for you.
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