The instructions you try to follow install Dovecot as POP3 server, listening on port 110. If you can't access port 110, this means that Dovecot is not running or is having problems.
Check if a dovecot process is running. Use the ss, netstat or lsof commands to check if pop3 or 110 are listened to. If dovecot is running, find its log messages (see
https://wiki2.dovecot.org/Logging).
And double-check your config files.