I tried searching the forums for this same issue, but couldn't find anyone experiencing the same exact thing as me.
I have a TCL P605 purchased in 2017, and I can no longer adjust the backlight or the local contrast. The TV stays exactly the same brightness whether the backlight is set to 0 or 100. The local dimming also does not work, and acts as if local dimming is off, even when it is set to high. It is like as if the TV backlight is stuck to 100% brightness with local dimming turned off, no matter what settings I choose. This is the same for all inputs and apps I have tried. This problem also exists when watching HDR and Dolby Vision content, local dimming will not work.
This TV was turned off and unplugged for a month since the last time I used it. It was not moved during that time, just sat on a TV stand unplugged for a month.
Troubleshooting steps I have taken:
Checking and installing updates.
Factory resetting the TV.
Forcing the software update to be installed again through the first secret menu.
Factory resetting the TV a second time.
Turning off fast boot and the 4hr power saver.
Another wierd thing, is that when I go to system settings and do a system restart, the TV will restart, come up with the red TCL 4K boot screen, but once it loads the Roku OS the screen turns off but the TV continues to work. I am able to navigate around the menus using audio cues, and was even able to start Netflix and get a video to play with the screen turned off, but only audio was working. And the screen is actually powered off, the backlight isn't on, so it's not acting like as if its turned on but not getting a signal, which is a problem I have seen other users report.
However, if press the power button to turn off the TV and then turn it back on, the screen comes on then. It just wont come on after doing a system restart. And of course the backlight control and local dimming still do not work after a restart.
If anyone has any other recommendations I can try, I am open to all suggestions!
EDIT: Forgot to mention the software version. The TV is running 9.0.2 build 4035.