If you try to access a remote database with nginx or httpd server, you will have a connection problem with error code (13) even if mysql/mariadb client works well in command line. The reason for that is a security limitation set by default not allowing httpd servers sur connect a remote DB.
To disable this limitation and make all work, just type in the command line :
# setsebool -P httpd_can_network_connect 1
You can list all the existing flags for httpd by using getsebool
# getsebool -a | grep httpd
In case you expect SELinux to be the cause of your problem, you can enable / disable it with the following functions
# setenforce 1 # setenforce 0
If you need to check what is the permission missing for SELinux you can run the audit
# tail -100 /var/log/audit/audit.log | audit2allow