I just installed TCL ROKU 40" TV.I have to turn it off and on manually.Remotr control does nothing.

If you're unable to change the backlight brightness on your TCL Roku TV and the backlight remains on even when the LCD screen is turned off, there may be an issue with the TV's firmware or hardware. Here are a few things you can try:

1. Reset the TV: Sometimes resetting the TV to its default settings can fix issues like this. To reset your TCL Roku TV, go to Settings > System > Advanced System Settings > Factory Reset. Note that this will erase all your settings and preferences, so you'll need to set them up again.
2. Check for updates: Make sure your TV's firmware is up to date. Go to Settings > System > System update and check for updates. If there's an update available, download and install it.
3. Check the cables: Make sure all the cables connected to your TV are securely plugged in. If the backlight issue started after you took the TV apart, it's possible that a cable came loose or wasn't plugged in correctly.
4. Contact TCL support: If none of these steps work, it's possible that there's a hardware issue with your TV. Contact TCL support for assistance or consider taking the TV to a repair shop.

Question edited for clarity 'make model (what it is)'.

How to Fix TCL Smart TV Remote - Support.com TechSolutions

https://www.techsolutions.support.com > how-to > how-...
Learn to solve common troubles you may have with your standard infrared and enhanced TCL Smart TV Remote.

TCL Roku TV: Wifi Internet Not Connecting? Try this First!!

https://www.youtube.com > watch
PREVIEW
2:47 I show you how to fix a TCL Roku TV (smart TV) where its not connecting to the wifi internet network (No Connection). By the end of this ...
YouTube · WorldofTech · Sept 16, 2020
Missing: 65R625 ‎' Must include: 65R625

TCL TV WiFi Not Working (Disconnects/Won't Detect/Connect)

https://readytodiy.com > tcl-tv-wifi-not-working-discon...
If your TCL TV WiFi won't turn on, press the 'Home' button on your remote control > press the right arrow button and select 'Network' > press the right arrow ...
Missing: 65R625 ‎' Must include: 65R625

Program Your Cable or Satellite Remote Control to Work with Your TCL Roku TV
Providers Codes for TCL Roku TVs Programming Instructions Comcast Cable (Xfinity) 11756, 12434, 12290, 12292 Comcast Cox Communications 1756, 11756, 12434 Cox DISH Satellite TV 1756, 535, 645 DISH Network Frontier Communications 688 Frontier Communications https://support.tclusa.com/tv-remote-codes/roku-tv-remote-codes

Have a look at the videos here...
https://www.youtube.com/results?search_query=TCL+Roku+TV+model+55S401TAAA

Regards
Geoff

To enjoy the streaming capabilities of your TCL Roko TV, it should be connected to the Internet. This can be done via WiFi (available on all TCL Roku TVs) or Ethernet cable (applicable only for TCL Roku 4K TVs). Here's a quick video that you can connect your TCL Roku TV to your home wireless network. You can also mention the step-by-step instructions given below, choose your preferred connection. For more information visit our site: http://www.rokucomlink.info/

Try resetting the Wi-Fi modem and tv

Find the code for your TV followinf instructions in n user's manual
If you don't have the manual for ypur unspecified remote, try to find it on line, using google search for terms

Manual, make and model of your remotr

Remotes One For All Global