New phone, never activated, unable to move to another carrier despite removal from account

I have a Google Pixel 6a that I'm trying to give to my grandmother. It was an extra phone Xfinity made me get (for "free") in order to save on my overall internet package a couple years ago, and I didn't want it and never opened the box or activated the number. My grandmother lives across the country from me with my aunt and needs a phone. My aunt doesn't have the same carrier as me, and she's adding my grandma to her account as a new line and phone (NOT keeping the same number I had). However, when she tries to put in the new SIM card, the phone says, "SIM card isn't supported, Ask Xfinity Mobile if this device can be unlocked. Tap here for contact details." I called Xfinity multiple times about this and since the issue arose, I've activated and deactivated it with Xfinity Support's help, removed the number AND phone from my account, and the latest Support agent I spoke with, put in a ticket to unlock the actual device itself. The email I got that was supposed to give me instructions on what to do next had no Unlock or Unfreeze codes (both said, "N/A"). They are STILL unable to activate the phone and I'm out of ideas. What is going wrong??? My aunt called her carrier and they confirmed they can support the SIM card. She has tried both the Xfinity and Mint card, and neither are working.