The security certificates that these computers had when they were last updated have expired, leaving no replacement. Older versions of macOS (appears to be 10.11 and earlier) do not have the updated certificates.
You can try installing the new certificate manually following the directions on this page:
Download the ISRG Root X1 certificate file from https://x1.i.lencr.org/
Open the Keychain Access app and drag that file into the System folder of that app.
Find the ISRG Root X1 certificate in System and double click on it, open the Trust menu and change "Use System Defaults" to "Always Trust", then close that and enter your password to confirm the change (if prompted).
Warning: Installing Root Certificates should be done with extreme care. Doing this with a malicious certificate would subvert the security process your computer employs, since it means an attacker could cause your computer to trust malicious websites pretending to be websites you already use.
Here is a page by Let's Encrypt verifying the URL above to download the certificate is trusted: https://letsencrypt.org/docs/lencr.org/