We are setting up the PROD environment in AWS and we need some help in choosing what option to go for database subnet group. We have 3 options :

1. Within non routable private subnet 2. Within routable private subnet 3. Including all subnets (default)

Firstly, I tested accessing the database which only had non-routable subnet and could not connect to the RDS (Postgres). Secondly, Although I was able to add the routable subnets later to the database, we wont be able to change the non-routable ip address assigned to the RDS later.

Will I need to go create the database on both routable and non-routable subnets as we need to be able to connect to the database from other applications? Or from the options I have given, which is the best one?