ALL LG devices from AT&T and Criket Wireless support firmware Over The Air Software Update only, So You can not download stock kdz firmware for phones with IMEI.
In this post, we will share with you the guide on how to dump Cricket LG Harmony (M257) software (ALL parititions) with LG UP TOOL, and You can use the DUMP files to restore your phone back to stock.
1) Install LG USB drivers
2)Install LG UP
3)Install LG G6 DLL
4)After install the LG G6 DLL from above link. you need also download
https://mega.nz/#!DsVlgBDL!SIk2qQJWl6Csi4Br0UBQQKWVux7E5zfIM-INkzmrKOg
rename the LGUP_G6.dll to lgup_common.dll and copy it to
rename the LGUP_G6.dll to lgup_common.dll and copy it to
C:\Program Files (x86)\LG Electronics\LGUP\model\Common
Now you have everything ready, put your phone to download mode and then run the L GUP.
On LG main screen, Click ‘DUMP”, Click start, From the pop up Partition List, Click Select all, and then scroll down to uncheck the users partition.
Click ok to start the DUMP process.