A company operates an Amazon RDS for MySQL database instance within a VPC that is configured to be isolated from the internet, both for inbound and outbound traffic. A security engineer is tasked with automating the rotation of database credentials using AWS Secrets Manager. Due to organizational security policies, the engineer is prohibited from using the default AWS Lambda function provided by Secrets Manager for this purpose. Consequently, a custom Lambda function has been deployed within the same VPC to manage the rotation of secrets in Secrets Manager. The engineer has also adjusted the security group of the RDS instance to allow connections from the custom Lambda function. Despite these measures, the Lambda function is unable to successfully communicate with Secrets Manager to perform the secret rotation when invoked. What corrective action should the security engineer take to ensure the Lambda function can effectively rotate the secret in Secrets Manager?