If your phone is network locked, then flashing a new ROM will not unlock it, as it is the IMEI that is network locked, not the firmware on the phone.
I got my phone from Vodafone UK and unlocked it the next day using a code from www.unlockunit.com
. Cost about $25. I did not have to root / flash the phone, as I simply put in a SIM card from a network other than the one my phone is locked to and the phone asked me for an unlock code.
I have no idea if ATT firmware will stop working once you unlock your phone. but I can confirm that the Vodafone UK firmware on my phone is still working fine.
Hope this helps. If so, please hit thanks button
Don't be shy to hit that 'Thanks' button
Samsung Galaxy S5 G900F - Stock 4.4.2
Samsung Galaxy Camera GC110 - Stock 4.1
ASUS Transformer Book T100 - Win8.1 Pro
Retired / Sold -
Galaxy Note 3 | Nexus 5 | Nexus 4 | Lumia 925 | Galaxy S3 | HTC Desire | HTC Desire HD | Galaxy S2 | Galaxy Nexus
Galaxy Note 10.1 (2014 Edition) | Nexus 7 (2013) | Nexus 7 (2012) | Galaxy Note | Galaxy Tab | Archos 80 G9 | Asus Transformer | Galaxy Note 10.1 GT-N8000