I made the following modification at mosquitto.conf (I indented the listener). The devices are connecting but mqtt is not listening to the messages:
pid_file /var/run/mosquitto.pid
persistence true
persistence_location /var/lib/mosquitto/
log_dest file /var/log/mosquitto/mosquitto.log
include_dir /etc/mosquitto/conf.d
password_file /etc/mosquitto/passwd
allow_anonymous false
"# listener 1883 0.0.0.0