connect to mobile data with Xiaomi phone

Sometimes Att help should be called ATT no help at all.

That brand of phone cannot be used on AT&T network because it dropped the 3-g for phone calls. Do not even try to use it, do not put a SIM card in it, you will just get service suspended immediately.

This is the whitelist of unlocked phones which are capable of being used on AT&T

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.att.com/idpassets/images/support/wireless/Service-Capabilities-Unlocked-Devices-ATT-Network.pdf&ved=2ahUKEwiImeqyqtPwAhW9KFkFHZXWAoIQFjABegQIGxAC&usg=AOvVaw1C5znQishzVLbCB1H7kNIA

Hi @jpsbeer,

Thanks for reaching out! 

We encourage you to check the support page which goes over how to access our network, and which bands are required to do so. You'll also want to check your device to see which bands are supported there as well. Other requirements will be listed on the page.

Let us know if you have any additional questions!

Collin, AT&T Community Specialist