Blocking access from a wordpress website to an HTML (Grafana Panel) code on my EC2 instance.

Hi - It seems like a networking issue since the EC2 instance is only refusing access, when it is requested from a wordpress website which may be coming from a different networking settings. Seems it is being able to access from a different computer which you can compare. Please check the following for the troubelshooting

1. The instance's IP address or hostname is correct.
2. The security group of the instance allows incoming traffic on required port
3. The network ACLs of instance subnet allows incoming traffic on required port and allow port for the outgoing traffic.
4. The route table of the instance’s subnet is configured properly to provide connectivity between EC2 instance and the client
5. There isn't a firewall blocking the connection

Hi. Security controls to consider advise that Amazon EC2 instances should not have a public IPv4 address, so first step suggested is to publish your Grafana dashboard using a Load Balancer and consider implementing a Web Application Firewal to mitigate application level risks.
This means EC2 does not need to be public, you can configure security groups to give access from the Load Balancer to your instance. The URL you will embed on your HTML will be the one created by the Load Balancer, this might require to check that your dashboard is available with alternat hostnames (this is to be checked on your EC2.