Hi Omar, thanks for posting your query and sorry I couldn't respond earlier as I had some emergency at home.

Well it seems, you need to create a private RDS instance and use its private IP which will be taken from the subnet you deploy the RDS into and then have a NLB created and point the target group to RDS IP address. If you have a public RDS instance then also you would be able to get the private IP of the RDS. Let me know if this solves the problem