[LG V20 Questions & Answers] MDMAchine:
Here since your on TMO, I compiled this for a buddy of mine through various sources (links provided) on the whole procedure start to finish. Might as well paste it for ya. Props to all original authors, etc...

I just did this on mine, same model, on firmware 10j. If your not on that or i step one will be to flash to the appropriate version. I didnt include that in my notes as myself and my buddy were both on 10j. But theres plenty of threads for that. if your on that, continue on:

Also if you are already flashed and rooted, jump to step #8 for the info regarding OTAs...

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Specifically for T-Mobile LG V20 H918

Firmware(s) 10I & 10J

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BOOTLOADER

SOURCES:

*** https://xdaforums.com/v20/how-to/guide-unlock-bootloader-t3488878

*** https://xdaforums.com/v20/development/h918-recowvery-unlock-v20-root-shell-t3490594/page2

*** https://xdaforums.com/showpost.php?p=69897433&postcount=1060

*** https://xdaforums.com/pixel-xl/how-to/index-google-pixel-xl-t3466185/post69239012#post69239012

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Warning: This will delete all your data. You'll also see a large warning every time you turn on the device (attached), this disappears in less than a second.

*Turn on developer mode:

Settings -> About device -> Software info -> Build number. (tap 7 times until it's enabled)

*Turn on OEM unlock and USB debugging:

Settings -> Developer options -> OEM unlock & USB Debugging. (turn it on)

Don't ever turn OEM unlock or Developer options off when using a custom ROM or recovery. This could lose to loss of all your data.

*Install LG drivers

WIN:

http://tool.lime.gdms.lge.com/dn/downloader.dev?fileKey=UW00120120425

MAC:

http://tool.lime.gdms.lge.com/dn/downloader.dev?fileKey=UW00320110909

You will also need adb and fastboot. You can download them in a portable small form factor here:

http://xdaforums.com/android/software/host-tools-t3402497

You may need to get an additional .dll for adb to work, if so you can pull it from the pixel add compilation here:

https://xenserver.underpants-gnomes.biz/~romracer/fastboot_adb_pixel.zip

*In Terminal navigate to ADB root:

Type

adb devices

and authorize your computer on the phone

Type

adb reboot bootloader

Type

fastboot devices

and make sure your phone shows up

Type

fastboot oem unlock

Type

fastboot getvar all

Should say (bootloader) unlocked:yes

Type

fastboot reboot

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TWRP & ROOT:

SOURCES:

http://www.droidviews.com/install-twrp-root-t-mobile-lg-v20/

https://xdaforums.com/v20/how-to/instruction-to-root-h918-10i-t3536472

https://xdaforums.com/v20/development/h918-recowvery-unlock-v20-root-shell-t3490594

https://build.nethunter.com/android-tools/dirtycow/arm64/

https://github.com/jcadduono/android_external_dirtycow#running

https://build.nethunter.com/test-builds/twrp/lge/twrp-3.0.2-1-h918.img

https://download.chainfire.eu/supersu

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*Prerequisites:

Your LG driver must be up to date. You can have problems with USB 3.1 if so grab latest drivers.

ADB installed, put all 4 recowvery files into the folder:

https://build.nethunter.com/android-tools/dirtycow/arm64/

Download the TWRP “twrp-3.0.2-1-h918” image:

https://build.nethunter.com/test-builds/twrp/lge/twrp-3.0.2-1-h918.img

Rename TWRP file to twrp.img and put it into internal storage (sdcard)

Download SuperSU:

https://download.chainfire.eu/supersu

Copy SuperSU onto external memory card (or keep in root of adb, and you could push it back, after formating).

You must be on a 100% stock ROM. Rooted or not.

USB Debugging & OEM unlock allowed in Developer Settings

Bootloader unlocked

Make sure all security/locks are off. No pins, fingerprint, etc…

Steps:

1. Plug your phone to your host computer. Make sure it's in MTP mode.

2. Open your command prompt:

cd\
cd (right click your mouse and paste the ADB platform-tools address, if using portable pack, where you put the adb folder)

adb devices

This will show your connected phone

3. Enter the following prompt: (you can simply highlight, copy, right click on command prompt and choose paste):

adb push dirtycow /data/local/tmp
adb push recowvery-applypatch /data/local/tmp
adb push recowvery-app_process64 /data/local/tmp
adb push recowvery-run-as /data/local/tmp

adb shell

cd /data/local/tmp
chmod 0777 *
./dirtycow /system/bin/applypatch recowvery-applypatch

* On ADB shell mode, you should see $ on the front. Wait for few…

./dirtycow /system/bin/app_process64 recowvery-app_process64

* Your phone screen may look weird. Wait for another few minutes. Once finished exit.

exit

4. Type:

adb logcat -s recowvery

You should see a lot of lines comes across your screen.

Once you see the ASCII box with the message about giving jcadduomo a hug you can press CTRL+C to exit logcat.

Reboot to the stock recovery:

adb shell reboot recovery

Restart a session:

adb shell

You will see a $ sign. Now to check…

getenforce

It should show Permissive. Thanks Dirty COW!!!

5. Temp Root. Lets patch the boot image:

cd /data/local/tmp
./dirtycow /system/bin/run-as recowvery-run-as
run-as exec ./recowvery-applypatch boot

Lets run as root

run-as su

You should have a #, indicating you have root. Ahh my little nix…

6. Flash TWRP

dd if=/sdcard/twrp.img of=/dev/block/bootdevice/by-name/recovery
exit
reboot recovery

The phone should have flashed TWRP and rebooted into recovery.

7. TWRP:

In TWRP swipe to allow modifications. Or TWRP will be replaced next boot!

To disable any encryption perform the factory reset, and the Format Data options.

Install SuperSU from the external SD location.

If you didn’t do a full wipe and reset, you probably want to wipe cache and dalvik.

Reboot to system.

8. Stop them OTA updates!!!

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https://xdaforums.com/showpost.php?p=72463487&postcount=4237

https://xdaforums.com/showpost.php?p=70795926&postcount=5

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Open up your dialer interface and type in 277634#*#

This will bring up a hidden menu and then select Wi-Fi test and then select OTA then select disable.

After that you will have to kill the hidden menu and you shouldn't be seeing the annoying OTA icon in your status bar anymore.

In file manager (root browser):

Rename otacerts.zip in /system/etc/security to otacerts.bak.

The full update is found in the cache partition called update.zip.

Move (or delete) the update.zip file from /cache directory (I put it on the sd card).

** dimm0k's method there seems to achieve a similar result, cant hurt to add the step of creating a zero length file after deleting/moving update.zip. I hadnt seen it, thus never did that but I havent had any OTA activity in a few days on mine.

Also in titanium backup (buy it already if you haven’t lol), freeze:

FOTA Update 7.0

Update Center 5.30.12

(I also disabled the com.lge.updatecenter.xxx overlay and themes, cuz whatever... lol)

Reboot and swipe away the notification if its still there. Should have stopped by this point.

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There ya go, good luck!