From my research, the AT&T E6820 is the most difficult to work on. The E6810 will at least go into bootloader mode. I've tried Kingroot and also its PC/USB version without success on my E6820. I have learned that you need to have Developer Mode enabled & select "OEM bootloader unlocked" in settings for the E6820. I'll post on the DuraForce Pro threads here if I have any success with ADB or fastboot.