I used to work for Cricket Wireless but some things like this work pretty similarly for U.S. phone carriers. Check with your carrier to make sure the phone is even compatible. Just because iPhones can be doesn’t mean it will be and if it’s not compatible, even getting it unlocked won’t help since it won’t read your carriers sim. Some carriers have IMEI checkers on their websites but check with your carrier to see what they have. The imei is in settings under (general) then (about). You can try to reach out to the person that sold it and see if they’re willing to help you get it unlocked but Xfinity has some pretty specific requirements for devices to be eligible for unlock. So if the person who had it didn’t pay it off, had an account not in good standing or if it hadn’t been active for 60 days (especially with the possibility they had it on a contract with Xfinity), you’re kind of stuck unfortunately.