A DB subnet group is a collection of subnets (typically private) that you create in a VPC and that you then designate for your DB instances.
Now simply try to access your NAS using your domain/subdomain (example: subdomain.miketabor.com) and your connection is now secured and you can confirm by viewing the SSL cert in your browser. Below I’ve included some extra information based on the questions I’ve received in the comments and email.
Unless you are working with a legacy DB instance, your DB instance is in a virtual private cloud (VPC).
A virtual private cloud is a virtual network that is logically isolated from other virtual networks in the AWS cloud.
When the SSL is active on the page you are viewing, they can tell by checking the address bar at the top of the browser.
There should be a small padlock icon in the address bar area.