Unlock A Used Phone (Pixel 4XL)

Hello everyone-

I purchased a phone on ebay which is locked to Xfinity.  I do plan to use this phone on a new second line for my fiance on my Xfinity plan and the IMEI checker says that Im good to activate.  The issue is that I would like to unlock the phone so we could use it while traveling.  I spoke to an agent on the chat and they said that it would be impossible to unlock this phone unless I was the original owner.  Is this truely the case or will I be able to unlock the phone after activation?  Ive reached out to the ebay seller to see if they can help, but who knows if that person is even the original owner or if they still have an xfinity account (another requirement I was given by the chat agent).  I'd just like to know if this is a lost cause and if I should pursue a refund.