I recently downgraded my EC2 instance. I can no longer connect to RDS. I think it might be that the internal IP is different and now the logins are attached to that specif
It sounds like your RDS is within a VPC inside a private subnet with security group and ACL. The only way to solve your issue is to take a snapshot and cerate a new DB instance out of it within the default VPC where all connections are allowed. After that you take backup classic backup using a db client or CLI.