Terraform-RDS Aurora-VPC - different VPCs error

Terraform
1

I’m trying to create an AWS RDS Aurora with Terraform and other EC2 instances. The EC2 instances are being directed to the VPC with no issues but the RDS can’t join the VPC due to The DB instance and EC2 security group are in different VPCs .

What am I missing here?

I have created the following:

variable "vpc_cidr" {
  description = "CIDR for the VPC"
  default = "10.0.0.0/16"
}

resource "aws_vpc" "vpc" {
  cidr_block = "${var.vpc_cidr}"
  tags = {
    Name = "${var.env}_vpc"
  }
}

resource "aws_subnet" "vpc_subnet" {
  vpc_id = "${aws_vpc.vpc.id}"
  cidr_block = "${var.vpc_cidr}"
  availability_zone = "eu-west-1a"

  tags = {
    Name = "${var.env}_vpc"
  }
}

resource "aws_db_subnet_group" "subnet_group" {
  name        = "${var.env}-subnet-group"
  subnet_ids  = ["${aws_subnet.vpc_subnet.id}"]
}

resource "aws_security_group" "RDS_DB_SG" {
    name = "${var.env}-rds-sg"
    vpc_id = "${aws_vpc.vpc.id}"
    ingress {
        from_port = 3396
        to_port = 3396
        protocol = "tcp"
        security_groups = ["${aws_security_group.BE_SG.id}"]
    }
}

resource "aws_security_group" "BE_SG" {
    name = "${var.env}_BE_SG"
    vpc_id = "${aws_vpc.vpc.id}"

    ingress {
        from_port = 80
        to_port = 80
        protocol = "tcp"
        cidr_blocks = ["0.0.0.0/0"]
    }
}

resource "aws_rds_cluster" "rds-cluster" {
    cluster_identifier = "${var.env}-cluster"
    database_name = "${var.env}-rds"
    master_username = "${var.env}"
    master_password = "PASSWORD"
    backup_retention_period = 5
    vpc_security_group_ids = ["${aws_security_group.RDS_DB_SG.id}"]
}

resource "aws_rds_cluster_instance" "rds-instance" {
    count = 1
    cluster_identifier = "${aws_rds_cluster.rds-cluster.id}"
    instance_class = "db.r4.large"
    engine_version = "5.7.12"
    engine = "aurora-mysql"
    preferred_backup_window = "04:00-22:00"
}
2

You need to add the aws_db_subnet_group to aws_rds_cluster.

Otherwise AWS will deploy your aurora cluster in the default vpc