My Apple TV remote will turn off my LG TV but will not turn it on

read Control your TV or receiver with your Siri Remote or Apple TV Remote, especially paying attention to this part:

Make sure that HDMI-CEC is on

If some features don't work, check the following:

1. Make sure that your TV or receiver offers HDMI-CEC support. For help, ask the TV manufacturer or check the manual.
2. Check that HDMI-CEC is turned on in the menu of your TV or receiver. Use the remote that came with your equipment. Since manufacturers often have different names for HDMI-CEC, look for a setting that ends in "Link" or "Sync." Here are some examples:

• LG: SimpLink
• Philips: EasyLink
• Samsung: Anynet+
• Sharp: Aquos Link
• Sony: BRAVIA Sync