Confirm that iPhone 8 a1863 works with AT&T?

I purchased an iPhone 8 (a1863) off of eBay. It was described as brand new and unlocked, but when I attempted to insert an AT&T SIM card, I received the following message:

SIM not valid

The SIM card that you currently have installed in this iPhone is from a carrier that is not supported under the activation policy that is currently assigned by the activation server. This is not a hardware issue with the iPhone. Please insert another SIM card from a supported carrier or request that this iPhone be unlocked by your carrier. Please contact Apple for more information.

Am I receiving this message because the phone isn't actually unlocked, or because it's an a1863, which is not actually compatible with AT&T?

This chart (iPhone - View countries with supported LTE networks - Apple) seems to indicate that the a1863 will not work on AT&T, but https://www.whistleout.com/CellPhones/Guides/Will-my-iPhone-8-8Plus-work-on-anot her-carrier and https://swappa.com/blog/unlocked-iphone-8-compatibility-guide/ seem to indicate that the a1863 should work on any network.

Does anyone have an a1863 that is confirmed to work on AT&T? Thanks!