I don't understand why my unlocked MotoZ (the one with the projector attachment, camera etc) isn't able to be used?

I spent a crazy amount of money on this phone in 2017 (cash) and even ran it on Metro PCS, yet when I enter my IMEI number here it comes back with an error. I really would like a one-year free subscription with my very expensive phone. What I really want to use this phone for is for special memories because I have the projector attachment also. And when I say it's a projector attachment I mean we're talking like I can fill up a wall or part of my ceiling with this attachment. I got it specifically for the projector. Unfortunately the screen OLED (or is it OLEM?) is so delicate that I had to have her replaced after the first week when I dropped it. It was such a new phone that they didn't even have a phone case for it at Best Buy like they do now. from my understanding my phone is a completely GSM unlocked phone not linked to any carrier and can be ran by Best buy or Verizon and to my further understanding Verizon is linked to Comcast. so if this Best Buy phone can be run by Verizon then shouldn't it be able to be used on Comcast / Xfinity.