I had a similar issue and wanted to add my findings to hopefully help those in the future reading this note. I went through all the above checks with ports, firewalls, TPH, etc and the new host still failed to register. Finally during some checking, I noticed the /etc/hosts file had a different name than what was in DNS for this host IP(the server was built with a different host name, then renamed without changing /etc/hosts). Since DNS resolved to the correct IP all the port checking worked. However, I assume when the ambari-agent was started it read the hosts file so it knew itself as the hostname in the /etc/hosts file which was incorrect. After updating the /etc/hosts file and restarting the ambari-agent everything worked.
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