I have faced this issue before, but sometimes, it's a fault from your device, not really from the server or the SSL certificate itself. I realized if the date and time of the device are invalid, it can lead to this error. Or if some application block the SSL connection, this error message will show up. So make sure to check the date and time of your device first. In the next step, temporarily disable/turn off any antivirus software to verify the issue.
Source: https://bytebitebit.com/687/your-connection-is-not-private/