ISPconfig checks if a pop3 and imap service is listening on the default port on localhost. If you disabled pop3 or imap in dovecot.conf, then ther is no such service at localhost and the monitor is showing it as offline.
Reconfigure dovecot.conf and set it back to
protocols = imap imaps pop3 pop3s
If you want to disallow connections to pop3 or imap, then close the ports in the firewall.