Ok! so firewalld is not running. What about iptables just to be sure? What’s shown if you run: systemctl status iptables
If it says not-found… try to check if you can reach 5900 port on the node from the frontend. You can install nmap in the frontend: yum install nmap and try to run: nmap -p 5900 X.X.X.X changing X.X.X.X with the IP of your node, let’s see what happens.
Cheers