Your connection is not private on iPhone
Why it happens on iPhone
On iOS the warning is usually the network, not the site: hotel, airport and café Wi-Fi intercept HTTPS at the captive portal before you have signed in.
The fix on iPhone
Open any plain http:// address to force the sign-in page, complete it, then reload. If the warning appears on mobile data too, the certificate itself is the problem.
Is Your connection is not private happening on your domain?
Running the check from outside your device separates a real certificate problem from a local clock or network issue in one step.
Free, no signup. Runs from outside your network, which is the only way to tell a server fault from a local one.
If that didn't fix it
iPhone has its own failure mode, but Your connection is not private has a wider set of causes that apply everywhere. The most common one overall is: the certificate expired (net::err_cert_date_invalid).
All causes and fixes for Your connection is not private