Is there a way to enable USB debugging from fastboot? I have a Nvidia Shield Tablet that froze on me. After powering down it hangs on the nvidia boot up screen when I try to boot it up. Nvidia support says it is a 'soft-brick', as I can't get into recovery mode from fastboot. I am trying to get a recovery image on the tablet. However, the bootloader is locked and I'm pretty sure USB debugging was not enabled ahead of time. Therefore, I can't perform an unlock using adb, and so can't flash a recovery image. When trying to access through a platform-tools terminal, it doesn't show in a list of adb devices. 

I'm pretty confident I have the correct drivers installed on my windows PC, which is connected to the tablet via USB. 

Nvidia Shield Tablet Wifi (2014 version)

Any help is appreciated at this point, I can't think of anything else to try. Sorry in advance for mixing up terms etc. I've just been researching the past few nights and trying to find a solution. I'm far from an expert.

I've tried these links, here and here but they seem to require USB debugging enabled or ability to access recovery mode from fastboot.

Thanks,
adb and fastboot are different modes. you can enter recovery without usb debugging. Press and hold Power + Vol Down. This is the bootloader screen. Now navigate to recovery with Vol Keys, Power to confirm. Wait a Moment. When the android symbol appears, press Vol Up + Power to enter recovery. Now do Factory reset.  

If device is locked you may not be able to enter recovery. you can unlock it from fastboot and try again. Unlocking will factory reset your tablet. Download platform_tools from google. Enter fastboot mode from bootloader screen. Connect the usb cable. Install the drivers. Open the cmd.exe as administrator. navigate to the folder. check with fastboot if the serialno appears. unlock.  

fastboot devices
fastboot oem unlock


then boot TWRP and see if partitions can be mounted. at least /system and /cache should be mountable, otherwise something is wrong. /data is encrypted but should be mountable too. for more info see here. Do not flash TWRP to recovery partition, first backup stock recovery!

fastboot boot twrp-3.3.1-0-shieldtablet.img


in twrp, there is a option reboot recovery which should you bring to recovery mode. if not, you still have to option to backup/wipe partitions from twrp. start with wiping cache. if phone is still not booting, reboot into twrp and format data. if any error occurs, you can pull the log from adb at any time. post the log file in the twrp support thread and ask for help. try another twrp version (3.1.1-0) depending on your android version

adb pull /tmp/recovery.log C:\Users\Admin\Downloads\platform_tools\recovery.log


if you need to enable usb-debugging from recovery, you may modify /system.
(but also the other methods you linked above should work)

pull the /system/build.prop and edit the following lines with Notepad++ (make a copy for backup)

persist.service.adb.enable=1
persist.service.debuggable=1
persist.sys.usb.config=mtp,adb


mount /system writeable, then push it back to phone. if you don't want to modify /system you can try to push it /data/local.prop (but i guess it is not supported on this rom) or do create a file /data/property/persist.sys.usb.config (i don't have exactly read this posts above)

adb push "C:\Users\Admin\Downloads\platform_tools\build.prop" /system/build.prop


Now place the RSA-Key somewhere on the phone and merge it into adb_keys

adb push "C:\Users\Admin\.android\adbkey.pub" /tmp/adbkey.pub
adb shell
mkdir -p /data/misc/adb
cat /tmp/adbkey.pub &gt;&gt; /data/misc/adb/adb_keys


if root access is required, you can change ro.secure=0 in build.prop too (but not recommended)