The RDS Instance that you have launched, does not have public accessibility. Please check below:
To give it the same, while launching RDS click on the following options.
1. Click on the following option:
2. Make your RDS Publicly Accessible
In case you want your RDS with no public accessibility, then it will only be accessible inside the same VPC where you have launched your RDS.
If you already know the above steps, still are getting problems, please check your Security Group of the RDS instance, to verify if all the settings are correct.