How to unlock iPhone 8 plus

"Unlock" may mean:

• Passcode unlock. If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support
• Activation unlock. Turn off Activation Lock - Apple Support
• Cellular carrier unlock. Only the original cellular carrier may do that if they choose to do so.
• Something else ...

You are wasting your time calling Apple, the phone is locked to xfinity, not to Apple. Apple is not a carrier. xfinity has to start the process and contact Apple after they approve the unlock. Then they contact you saying it has been unlocked. At that time (after doing a backup) you restore your iPhone as a new device and it will say, "Congratulations, your iPhone is not unlocked". Then restore again from your backup.

https://www.xfinity.com/mobile/support/article/how-to-unlock-iphone

Note that xfinity is even wrong in this article, just putting a new sim in it will not unlock it.