I was having the exact same issue, cannot hit the CM login using public DNS or IP on port 7180.
Following steps will help you :
iptables stopped (service iptables stop) SELinux disabled (got to /etc/selinux/config and disbaled the selinux) curl/wget localhost:7180 works (check the curl status) ufw allow 7180 service httpd status should be running. check va/log/cloudera-scm-server log : if any error found then troubleshoot the error cloudera-scm-server status (should be running state) netstat -nap | grep 7180 returns (if running other service then kill it)
telnet localhost 7180 (should be connected)