CRITICAL: All RDS instances in sa-east-1 not accessible externally - Infrastructure issue

Based on your description, this appears to be a connectivity issue with your RDS instances. While you've verified many important configurations, there are several potential causes to investigate:

1. Public Accessibility Setting: Check if the "Publicly accessible" parameter is enabled for each RDS instance in the RDS console under "Connectivity & security". This setting is required for external access, regardless of security group configurations.

2. Subnet Configuration: Even if your subnets are "public" (have routes to an internet gateway), RDS instances should ideally be placed in private subnets for security reasons. If your RDS instances are in a DB subnet group that contains both public and private subnets, they might have been placed in or failed over to private subnets during creation or maintenance.

3. Network ACLs: Verify that the Network ACLs associated with your RDS instance's subnets allow both inbound and outbound traffic on your database ports (5432 and 5435).

4. Route Tables: Double-check that the route tables for your VPC are correctly configured to allow traffic between external networks and the RDS instances.

5. Recent Maintenance: If this happened after a maintenance window, it could have changed some network configurations. AWS maintenance activities sometimes modify settings that can affect connectivity.

For immediate troubleshooting:

• Try connecting from an EC2 instance within the same VPC to determine if this is specifically an external access issue
• Temporarily modify security groups to ensure they're not the cause
• Check if there are any AWS service health dashboard notifications for RDS in sa-east-1

As a best practice for security, consider using alternative methods to access your RDS instances securely, such as through a bastion host or AWS Systems Manager Session Manager, rather than exposing them directly to the internet.
Sources
Troubleshooting connection issues to your Amazon RDS DB instance - Amazon Relational Database Service
RDS connectivity failure after AWS maintenance window | AWS re:Post
Troubleshooting connections to your RDS for PostgreSQL instance - Amazon Relational Database Service
Community | Connect securely to an RDS instance