What is the root cause of this problem?
The VPC peering connection has not been configured properly for the EC2 instance to communicate with the DB instance.
The route table of the private subnet that hosts the DB instance does not have a NAT gateway configured for communication with the EC2 instance.
The public subnet that hosts the EC2 instance does not have an internet gateway configured for communication with the DB instance.
The wrong security group was associated with the new DB instance when it was restored from the snapshot.
Explanations:
VPC peering is not necessary in this scenario because both the EC2 instance and the RDS instance are in the same VPC.
The private subnet where the DB instance is hosted does not need a NAT gateway for communication with the EC2 instance in a different subnet within the same VPC. Communication within the same VPC does not require a NAT gateway.
The public subnet hosting the EC2 instance does not need an internet gateway to communicate with the DB instance in a private subnet within the same VPC.
The issue likely arises from a misconfigured security group on the restored DB instance. If the correct security group rules were not applied, the EC2 instance would be unable to establish a connection to the DB instance.
I systematize that the answer is:
The wrong security group was associated with the new DB instance when it was restored from the snapshot.
If memory serves me right, the answer is:
The wrong security group was associated with the new DB instance when it was restored from the snapshot.