How to fix the RDS / MariaDb Connection issues ?

Hello.

If you want to check the connection from EC2 to RDS, I think the Reachability Analyzer function will be useful.
https://aws.amazon.com/jp/blogs/database/troubleshoot-network-connectivity-to-amazon-rds-databases-using-vpc-reachability-analyzer/
https://repost.aws/knowledge-center/vpc-connectivity-reachability-analyzer

If you are connecting directly to RDS from your local PC, the security group settings may not allow connection or the RDS public access settings may not be enabled.
Also, it is not recommended to make a database such as RDS public for security reasons, so I think it is better to change the connection method as described in the document below.
https://aws.amazon.com/jp/blogs/mt/use-port-forwarding-in-aws-systems-manager-session-manager-to-connect-to-remote-hosts/

Hi, You should follow the guidance of this knowledge article to verify multiple possible causes of such error: https://repost.aws/knowledge-center/rds-cannot-connect

Best,

Didier