So after much time trying to get an AWS ECS cluster to recognize my instances, I realized it was the because the cluster was in the default cluster for the account while the instances were in a custom VPC. I am able to get my instances registered with the cluster if I use the default VPC instead of the one I created so I know that's not the problem.
How do I specify that the cluster is in a created VPC rather than the default? Basically, I want my instances in "VPC-x" to get recognized/registered by the ECS cluster. Is that even possible?