It has been reported countless times that ATT WP devices except 920 and 820 can't be flashed with anything that is not intended to be loaded on them. For example, on ATT L925 you can flash firmwares with ATT L925 product codes only. If you try to flash T-Mobile's L925 product code firmware on ATT L925 won't work. There is a theory that you need to stay within the same RM-XXX number. Both ATT and T-Mobile L925 are with the same RM-XXX number, i.e RM-893 and yet the ATT L925 is fully locked and can't be flashed with anything. On the other side, T-Mobile's L925 can be flashed with any L925 firmware with exception of maybe the firmware for RM-910. Some folks have reported that they have flashed their T-Mobile L925's with firmwares intended for the European Union (and lost LTE since EU L925's use different LTE bands) and successfully flashed back to stock T-Mobile firmware.
The same goes for L1020. I wanted to flash Rogers L1020 on ATT L1020 and it didn't work. I think i've exhausted all combinations and followed all public tutorials on how to flash ATT L1020 ... Nothing works ...
Same thing for ATT L1520, ATT L520 ...