Factory unlocked devices appear to be getting less support with AT&T blocking access to WiFi calling and VoLTE unless you’re using AT&T sold and locked approved devices.
Of course - they don't make any money when you buy an unbranded phone. Ask your local store where the nearest Device Support Center is. Go there and ask if they'll help you, if you have any specific issues. (They're basically the "out there in the world" end of the tech support department.)