Older versions of iPhone OS aren't new enough to have modern certificate authorities trusted, meaning only sites using older certificate authorities are able to have a secure connection.

Here are links to modern Certificate Authorities that may not be trusted on older devices (some other complications may prevent sites using these certificates from working, those can not be fixed by these.)

The original certificates can be found here:

ISRG Root X1 CA (+Let's Encrypt): https://letsencrypt.org/certificates/

This site only exists to act as a way to download these certs on older devices that can't load the original webpages. Please try to remember that I'm a random person on the internet, and always be diligent before blindly trusting something like an SSL certificate from an unknown website.

Please link to the original certificate pages if helping somebody, and only link to this page if they can't get the original page loaded.