Try the following:
Turn off the TV if it is on and disconnect the power from the TV.
Remove the back cover from the TV and disconnect the cable between the power board and the mainboard.
Reconnect the power to the TV.
The TV backlights should turn on full intensity and stay on. There will be no images on the screen because the mainboard is disconnected.
If they turn on and stay on then there is a problem with the mainboard or the cable or cable connections between the mainboard and the power board.
Check the voltages at the connector where the mainboard cable was connected to to see if they’re as per what is shown on the power board.
Be safety aware as there is exposed lethal AC voltage on the power board
If they blink or don’t turn on then there is a problem with the power board or the connection between the power board and the LED strips
Disconnect the power from the TV and then disconnect the LED power cable from the power board. Reconnect the power to the TV and then measure the voltage at the LED power connector on the power board between each of the +VLED pins and earth. The voltage should read >220V DC. If there is this amount of voltage then there is a problem from the LED power cable to the LEDs or a faulty connection between the LED power cable and the power board