Connecting to instance in private subnet through Network Load Balancer

I have a NLB in public subnet with a static ip address and an EC2 instance in private subnet. Connection to EC2 is not working for TCP protocol on port 6000. I have a NAT gateway in public subnet and route table of private subnet is pointing to 0.0.0.0/0 and NAT gateway. EC2 security group, inbound connection is open for 0.0.0.0/0 on port 6000. NACL for both subnets allows inbound and outbound connections. Route table of public subnet has an IGW. What might be missing in the configuration to make it work?