(This is a draft. Will update with more info. Might help someone)
I wanted to install CyanogenMod as this AT&T phone still has Android 4.
Found instructions here: https://wiki.cyanogenmod.org/w/E970_Info.
I was not able to load the bootloader or recovery mode. Realized I needed to root the phone.
Rooting instructions (thread at xda-developers.com) were here and easy to follow and execute.
Installed FreeGee app then. Used FreeGee to boot into bootloader using TWRP. That worked.
fastboot did not work with my phone. adb worked but not fastboot
Found instructions online (http://developer.android.com/tools/extras/oem-usb.html) how to do it. It was my driver. Disconnected my phone. Got it into bootloader, then connected to PC via USB. In device manager found Other/Phone. Manually installed driver for this from
I now performed a fastboot flash per instructions from the cyanogenmod site
Moved the appropriate cyanogen zip file to the /sdcard/ folder on the device with adb push
Backed up my system with twrp backup (this was awesome as later I needed this backup).
Installed cyanogen. Rebooted and there it was. I was able to use the cyanogen apps. Android 5.0.2 was installed and working.
However, I needed my google apps. Found at least five different google apps (gapps) images online but none installed successfully. I would push the file to the Android device, then install the zip using twrp. Errors would be reported each time on installation, or upon booting the phone it would produce an endless stream of pop ups that google apps are not working properly. To recover, I would have to wipe everything using twrp, then install cyanogen 12 again from the zip file (somewhat of a hassle).
As of now I cannot find a working google apps image for this phone.