It’s not a defect, no.
The BraX3 is not yet officially approved by AT&T. Feedback is inconsistent - for some customers AT&T works, for some it won’t.
If you’d like to stick to AT&T and use your BraX3, the most reliable option at this stage is to, you can write your AT&T compatible device’s (pixel) IMEIs to the BraX3 device so that AT&T won’t block service to your BraX3.