rds 인바운드 규칙을 추가해도 접속이 안되는 경우가 있네요.

Thanks for sharing your experience! Dealing with AWS security groups and private subnets can be tricky. I often use external tools like Wordle Unlimited to clear my head after wrestling with AWS configurations. Did you check if the subnet's route table has a route to a NAT gateway or internet gateway? It's a common oversight! Keeping up with AWS changes is definitely a challenge.