Let me explain the process. I assume that you have a RDS instance (using mysql) with these properties:
I also assume that you created a database inside the RDS instance with the following properties:
(Change the properties to those matching your case)
First of all you would need to move the contents of your local database to RDS. You can use phpmyadmin to export it: https://docs.bitnami.com/aws/apps/testlink/#how-to-access-phpmyadmin
With this information, go to
/opt/bitnami/apps/testlink/htdocs/config_db.inc.php and edit the contents so it ends like this.
x.x.x.x must be substituted with the IP address of your RDS instance. With this, now testlink should work with your database.
Hope it helps.
Javier J. Salmerón