I have an Amazon Workspace running inside the Private Subnet. In the same subnets i have launched my RDS. I have give access to the Workspace security group to access the RDS inside RDS security group.
Even after all this configuration, i'm getting timedout error whenver i'm trying to connect to RDS from the workspace.
You must allow outbound access for the specific port on the security group attached to the WorkSpace and inbound access from the WorkSpace on the RDS security group (you can reference the security group IDs instead of ip-addresses).
Take also a look on the AWS Reachability Analyzer to check if everything should work. (use the network interface of your WorkSpace) https://docs.aws.amazon.com/vpc/latest/reachability/getting-started.html