Jenkins 2.89.3 "This Jenkins instance appears to be offline"
From your question, I assume that you used Internet connection to do a fresh Jenkins installation and the Jenkins service is started.
The error message points to SSL error. It seems that you have followed an answer in this forum Why does Jenkins say "This Jenkins instance appears to be offline", but with no success.
When I had similar exception message I managed to fix it after importing self-signed certificate into JVM (Jenkins uses Java).
keytool -import -alias $ALIAS -keystore $JAVA_HOME/jre/lib/security/cacerts -file $PATH_TO_PEM_FILEMore detailed information can be found here:https://support.cloudbees.com/hc/en-us/articles/217078498-PKIX-path-building-failed-error-message