Well there are a lot of factors that could be the cause. It could be the country/region, my account, when I bought the phone, which model I have etc. But one thing is sure. It does not work and I hate Google because of it. Non of the tutorials fixed it either. So for me it is server side problem as I have also completely removed YouTube with Titanium backup.
Tried to mod latest Youtube APK named 5.17.6 Google have made it all impossible to change something all methodnames and classnames are random alphabetical marks and file directory itself is a real mess, no potential by finding somethid through folders, ad removal also hardened through a method, after hours of searching I finally found line which was getting phone's capability to stream higher resolutions than 720p or not, but after a change of it's code verify error popped up because of other method checking the value change, thanks Google...
But for older Apps with normal structure I could enable screenoff, up to 4k streaming and adless.
I've resorted to using the desktop version if I want 1440p, but for most videos 720p is fine for me. I'm sure as more YouTube channels start using QHD and 4K google will let us increase the resolution in the app
1. Wipe YouTube app
2. Uninstall YouTube app
3. Open file explorer that has root permission (i.e. es file explorer)
4. Navigate to /system/ (Make sure it's mounted as r/w)
5. Open build.prop
6. Change the line at ro.build.version.release=4.4.4 to ro.build.version.release=5.0 and save
7. Reboot device
8. Download and install this YouTube version https://www.dropbox.com/s/vm9ncnzyy7g94pf/com.google.android.youtube-184.108.40.206.apk?dl=0
9. Go to the play store and disable automatic updating of the YouTube app
10. Open YouTube app and select a video that supports 1440p
11. Pause the video and change the quality to 1440p
12. Close the video (just the video, not the app)
13. Select the video again
I've made this guide as thorough as possible. The last few steps are optional and not always necessary, occasionally the setting for 1440p doesn't like to stick. A quick way to check if it took is if the load bar resets, if it does you don't need to close the video and reopen it.
So, I poked around some more and it looks like youtube was checking our build version in several places (both locally and at their end, ->server-side) I hooked all the checks I could find and rebuilt the .apk
This should work guys. I still have some problems with the signature, so you have to manually remove youtube from system/app and replace it with this one: Download Modded YouTube.apk
I tested this multiple times on two different Note 4's, with success. Hopefully third time's the charm. I really hope this one works for you
Instructions for those reading this the first time:
1. You Must be Rooted
2. Disable YouTube auto-updates from play store
3. Uninstall YouTube completely from your system
4. Check to see that there is nothing left of the old installation, both in the system/app and system/priv-app folders.
5. Push this Modded YouTube.apk to system/app
6. Reboot and load up YouTube. You should now have the option to watch videos in 1440p
I hated the fact that editing the build.prop broke S Note etc, so I patched the newest youtube 6.0.13 to send version 5.0 to the youtube server in order to enable 1440p without editing the build.prop
You will still need to completely uninstall/remove YouTube from your device and replace the YouTube.apk in system/app tho(remember to set correct permissions, reboot and disable auto update from play store).