Replacing the backlight is the fix. The issue is that the individual LEDs are starting to fail. As they do then their ability to produce white light gets screwed up. They go purple.
This is not a problem that can be fixed by pressing buttons. It's not an incorrect setting. Something is actually broken and the fix is to replace all of the backlights because trying to fix individual ones isn't practical and (even if possible) would result in different colour of white in patches on the screen. This would also affect the rest of the colours in each light zone compared to its neighbours too.
The answer is to replace all the backlight. That, or live with it until the backlights all go purple, then junk the TV or just sell the TV now to a repairer, but don't expect to get much for it (<£50) because the repairer then has to spend the dosh on a new backlight system before that can resell, and s/h entry-level repaired TVs don't fetch a fortune when new ones are as cheap as they are.
The reason your Technica lasted longer is partly it was a lower cost set - bet it wasn't 4K - so wasn't aiming as high, and secondly, it didn't have to support the huge cost base of LG so they weren't trying to cut as many corners.
Basic LGs came a cropper because the company was trying to out-do the budget brands. Running the backlights as hard as this just accelerated the wear.
