safari cannot establish a secure connection

Try this...

Go on the Apple certificate page:

https://www.apple.com/certificateauthority/

Download the Apple Intermediate Certificates

Apple IST CA 2 - G1 Certificate (direct download)

Double click the downloaded certificate to install it in Keychain Access.

You should see it now, in login certificates.

https://discussions.apple.com/thread/251211674?answerId=253087027022&page=1

You are attempting to access a website that is still using (now outdated) TLS 1.0/1.1 security protocols and Safari 14.0.3 is warning you of the fact that they are no longer secure.