Download and install the Firware packages for ATT:
source.sierrawireless.com/resou … ware-list/
install the firmware. reboot.
From the Ace Manager, Wan/Cellular tab(at top) -> Wan/Cellular sub tab on left -> under Network Credentials, click the [+] to open the list.
For ATT enter the apn: wap.cingular or try pta for LTE 4G
then reboot device.
should get connection, if not you’ll need to google search the AT&T apn.