1. Make sure your Date and Time zone is set properly if your computer is displaying the wrong time.
2. If you have installed and are using antivirus software that provides “HTTPS scanning” or “HTTPS protection”, you will see this error. I would recommend you disable your firewall or antivirus software.
3. If the problem still persists, you can try to disable SSL Error warnings. To do that, go to this tutorial:
After trying the above, reset your modem. You may have to call the provider to make sure your device is authorized. They can send a reset signal and check if your device is "online".
My provider accidentally deleted my cable modem from the account causing the "not private" message due to the tool they use to route the browser. Hope that works for you!