I can't connect my Galaxy S21 Ultra service - Claims it's uncompatible

So I actually purchased this phone through Xfinity and was using just mere months ago just fine. I recently upgraded to an iPhone 15 but after it broke multiple times, I decided to revert back to my trusty Samsung for the time being as I'm not forking over hundreds of dollars to fix the iPhone again. The screen won't even turn on. Now I have a new Sim card that I installed and everything but when I go to "Add mobile plan" it says "No plans found". I tried through the Xfinity app but it claims that "Unfortunately, your phone isn't compatible". I know it's a few years old but this doesn't seem right at all. Especially since I was just using it not that long ago. Is it really too old to use with Xfinity Mobile? This is ridiculous if true.