From a different resource I believe I can submit the answer. First I used several versions including the latest 19b and serval platforms.
I found the matlab documentation to be somewhat deficient. Additional arguments are required especially the secondary database name as "DatabaseName". i also noted that if example doccumentation is followed such that "DataSourceName" is distinct from "DatabaseName" that introduce a second connection.
Critical non-default AWS settings included setting public accessibly to true, at database creation asserting secondary database name to instantiate the database (a nearly hidden setting with unexpected behavior), and modifying security group inbound/outbound constraints to accept the IP of my desktop as a client.
I found this AWS resources to be particularly valuable despite it addressing an obsolete RDS dashboard.
Here is the working code:
%resource credential specification
username = "XXXX";
password = "XXXX";
databasename = 'myDB';
endpoint = 'XXX.us-east-1.rds.amazonaws.com';
connector = '/Users/XXX/Desktop/postgresql-42.2.9.jar';
opts = configureJDBCDataSource("Vendor","PostgreSQL");
opts = setConnectionOptions(opts ,...
"Server", endpoint ,...
"PortNumber", 5432 ,...
"JDBCDriverLocation", connector );
%database link confimation
[status,msg] = testConnection(opts,username,password);
%database reference establishment
dcon = database(databasename,username,password);