[G2 Q&A, Help & Troubleshooting] Turbine1991:
Okay, take it really slow. I've just completely rooted, put a custom recovery on and am now on Android N. There's many things which can go wrong - requiring you to restart the process.

Treat it as a D801, as the T-Mobile firmware booted. AutoRec does not root the device. Are you on KitKat or Jellybean? If you're on KitKat, do the following.

You must use IORoot to root the device - it does it in such a way it'll accept custom recoveries.

(You already have the phone's USB drivers obviously)

1) Download IORoot V25 from here.

2) Extract the file.

3) We need to unlock USB debugging, so make sure the phone is not connected to the pc. Settings -> About -> Software -> Tap on "Build" like 8 times fast. Go back into settings and you'll see "Developer Options", tick USB debugging. Now if your device freezes up like mine did when tapping build, then download an app which lets you go directly to developer options.

4) Hook the phone up to the PC, make sure it's not in lock screen.

5) Double click "root.bat" normally (not as admin). Keep going through this process until it brings you to the stock recovery area where you can flash over ADB.

6) It'll walk you through the process where it boots back into Android.

7) Open SuperSU, which it installed.

8) In the settings somewhere, make the grant always accept rather than prompt. My device was bugged not allowing root unless it was forcefully granted like this.

AutoRec - Recovery Install

1) Copy the contents "bumpboot-v1.0-blastagator-signed.zip" and "busybox.apk" and "flashify.apk" onto your device. Download here.

2) Download TWRP for T-Mobile 2.8.6.0 from here, extract recovery.img and copy that onto your device.

3) Copy AutoRec KitKat for T-Mobile here.

4) So you should now have "D801_AutoRec.apk", "recovery.img", "busybox.apk", "flashify.apk", "bumpboot-v1.0-blastagator-signed.zip" on your phone.

5) Proceed to install "AutoRec.apk", ticking unknown sources. Let it boot into recovery. Now reboot.

6) As my recovery didn't stick, install and open "busybox.apk", click install in the program itself. Install and open "flashify.apk", flash recovery, browse using flashify to the location of "recovery.img", let it reboot into recovery. This is where I had to flash "bumpboot-v1.0-blastagator-signed.zip", otherwise I couldn't boot into the OS. So flash that file.

7) Reboot back into the OS. Until we flash lollipop, there wasn't any other way for me to reliably get into TWRP recovery without repeating step 6.

8) Copy over the Bootstack and Stock Rom from here. Don't worry about SuperSU or anything else.

9) Repeat step 6 to boot back into recovery.

10) Wipe everything except internal memory in TWRP.

11) Flash the lollipop bootloader.

12) Flash the lollipop stock rom.

13) Reboot.

14) Now you're able to get back into recovery any time by holding the power + volume down combo, releasing when the screen goes on - and holding it down again.

15) Feel free to flash LineageOS 14.1 or anything afterwards. Make sure to backup your EFS partition at some stage before this - just incase you can't connect to cell towers in the near future.

As you can see, I had to improvise a bit. No guide was good enough to just work in 2017 for my D800. These should work fine for your D801.