Is this violating public accomondation codes?

[True Federalist (진정한 연방 주의자)]

While it may be the same phone physically on the outside, it probably can't use AT&T's frequency bands. Indeed that would be true for any Chinese phone on any U.S. (or European) carrier unless it was a more expensive global phone able to support the networks in multiple regions.

[True Federalist (진정한 연방 주의자)]

Edit: It's a phone purchased in China, not a phone made by a Chinese company. It's the exact same model of the same phone sold in the USA.

Just because it's the same model doesn't mean it can access the same frequency bands. China, North America, and Europe all use different frequency bands.