That's a known issue with recent versions of the U1 firmware. If you don't get the popup, it won't convertI've tried but I never get the pop up to install the carrier software when I insert the AT&T sim card upon reboot.
I removed my sim card before flashing... Flashed with XAA AP,BL,CP and non home CSC files (G988U1UES2DUE2). Upon reboot I inserted the AT&T sim card and its just connects to service, no pop up.