AWS made the announcement that the old certificates (rds-ca-2015) will expire on March 5, 2020. Using these old certificates may cause an interruption of connectivity between Deep Security Manager (DSM) and your Amazon RDS DB instances, if the communication between the Deep Security Manager and the RDS is encrypted.
By default, communication between the Deep Security Manager and the database is not encrypted. See this Help Center article for more details: Encrypt communication between the Deep Security Manager and the database.
In case the encryption between the manager and RDS is enabled, it is strongly recommended to follow the instructions provided by AWS to ensure your RDS has the latest CA certificates. For the detailed procedure to update RDS database instances, refer to this AWS document: Rotating Your SSL/TLS Certificate.