Wow. Years and years ago I was pretty current with linux server admin but then I drifted away. Well, I’m back to it.
Setting up a CentOS 8 VM for drupal admin on Azure. Had things working wiht a local MariaDB server but wanted to use a managed server so set that up and nothing worked when I switched the credentials over to the external server. A couple days of on-and-off-again fussing with it and I found this lucky StackOverflow post that saved the day even though it was keyed to nginx rather than Apache.
so.. the solution if you can connect to a mysql or MariaDB server using local php or the mysql command but your web served scripts give a permission error you might need to redundancy
setsebool httpd_can_network_connect_db on
To enable external access to external databases.