This is a gotcha but be aware sometimes iptables may be active and loaded by default.
Also make sure you don't just disable firewalld but also stop it otherwise it will still block stuff:
systemctl stop firewalld
If the above is not the issue then it is possible iptables is running and blocking stuff too, so you'll need to stop iptables.
So in addition to opening firewalld or disabling it, you would need to disable iptables........
systemd is like the service manager for your Centos and other modern Linux distributions (including Debian/Mint/Ubuntu) allows you to enable services, stop them, restart them, check their status and even reboot your system.
The key commands or arguments you will use with systemctl are the following:
Unit Commands:
list-units [PATTERN...] List loaded units
&nbs........