Upgrade iphone

I'v been a comcast/Xfinity customer for 20 years.  I have a 'by the gig' plan for 2 phones that is super cheap.  I want to upgrade one of my phones to a iPhone 17. I know I will need to upgrade to a more expensive current mobile plan.  But I was told that I am not eligible to upgrade and upgrade my phone because I don't have Xfinity Internet.  I live in a retirement community and they supply Xfinity Cable TV service, but the Internet comes from another provider.  I have no control on the Internet provider. My only choice seems to be to move all to Consumer Cellular or another provider.