DMZ not communicating with internal database

Did you try everything it lists? Firewall is open? That MySQL user has remote access permissions from the DMZ server's IP?

I would start with this article: https://www.jamf.com/jamf-nation/articles/135/troubleshooting-database-connectivity-from-the-jamf-pro-server

Specifically the section covering the DataBase.xml file. The file may not contain the password you used for your database. If it’s not that, make sure SQL is running and you’ve setup a MySQL user in the MYSQL command line and granted it sufficient permissions.