Everything in your post suggests that this is either a networking issue, or for some reason Splunk is not accepting connections.
The difference between the results of your browsers tests is just an artifact of how your browser reports failures for IP vs DNS name.
On your indexers, run netstat to confirm that the ports are open on 8089.
Confirm your SH can ping/route to indexers
Confirm you have no harware/application firewalls keeping connections out.
On centos 8, you can try disabling the Firewall temporarily systemctl stop firewalld to see if that resolves the issue - remember to restart it and add rules if it does!