For older macOS, try:
- downloading https://letsencrypt.org/certs/isrgrootx1.der
- Open the Keychain Access app and dragging that file into the System folder of that app.
- then 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).