You've created a network load balancer but it doesn't work (you can't reach your app on your EC2 instance). What might be a possible reason?

Missing security group or misconfigured one. For example, if you go to your instances in the AWS console you might see that the instances under your NLB are in "unhealthy status" and if you didn't create a dedicated security group for your NLB, that means that the security group used is the one attached to the EC2 instances.

Go to the security group of your instance(s) and enable the traffic that NLB should forward (e.g. TCP on port 80).