Timeout connecting to port in EC2 instance, probably ACL issue but can't figure it out

I'm having a lot of trouble connecting to an application running on my newly created EC2 instance from its public IP address. I'm able to log into the server via rdp, however. Once there, I can access the from http://localhost:8443.

To make sure the app wasn't the issue, I loaded a test TCP server (Hercules) and ran it listining on port 9999. I was also unable to connect from the Hecules client via its public IP address.

An nmap port scan shows neither the application port (8443) nor the Hercules 9999 port are available.

I am able to ping the server. Also, there's only one security group attached, which allows all IPv4 traffic, specifically:

type: all traffic Protocol: all Port Range: all Source: 0.0.0.0/0

This instance belongs to the same VPC and subnet as another server which I have had no trouble accessing. That server also also has the same security group)

The server's current IP address is 3.15.34.4,

What am I missing?