terraform.tfvars

os_user = "foo"
psk = "pass1234"
fritzbox_wan_ip = "1.2.3.4"
fritzbox_cidr = "192.168.178.0/24"

terraform.tf

variable "os_user" {
  type = string
}

variable "psk" {
  type = string
}

variable "fritzbox_wan_ip" {
  type = string
}

variable "fritzbox_cidr" {
  type = string
}

data "openstack_networking_network_v2" "networking_network_1" {
  name = "${var.os_user}-net"
}

data "openstack_networking_subnet_v2" "networking_subnet_1" {
  name = "${var.os_user}-subnet"
  network_id = data.openstack_networking_network_v2.networking_network_1.id
}

data "openstack_networking_router_v2" "router_1" {
  name = "${var.os_user}-router"
}

resource "openstack_vpnaas_ike_policy_v2" "vpnaas_ike_policy_1" {
  name = "${var.os_user}-ike-aes256-sha512"
  encryption_algorithm = "aes-256"
  auth_algorithm = "sha512"
  pfs = "group2"
}

resource "openstack_vpnaas_ipsec_policy_v2" "vpnaas_ipsec_policy_1" {
  name = "${var.os_user}-ipsec-aes256-sha512"
  encryption_algorithm = "aes-256"
  auth_algorithm = "sha512"
  pfs = "group2"  
}

resource "openstack_vpnaas_service_v2" "vpnaas_service_1" {
  name = "${var.os_user}-vpn-service1"
  router_id = data.openstack_networking_router_v2.router_1.id
}

resource "openstack_vpnaas_endpoint_group_v2" "vpnaas_endpoint_group_subnet_1" {
  name = "${var.os_user}-vpn-ep-subnet"
  type = "subnet"
  endpoints = toset([data.openstack_networking_subnet_v2.networking_subnet_1.id])
}

resource "openstack_vpnaas_endpoint_group_v2" "vpnaas_endpoint_group_cidr_1" {
  name = "${var.os_user}-vpn-ep-cidr"
  type = "cidr"
  endpoints = toset([var.fritzbox_cidr])
}

resource "openstack_vpnaas_site_connection_v2" "vpnaas_site_connection" {
  name = "${var.os_user}-vpn-conn1"
  ikepolicy_id = openstack_vpnaas_ike_policy_v2.vpnaas_ike_policy_1.id
  ipsecpolicy_id = openstack_vpnaas_ipsec_policy_v2.vpnaas_ipsec_policy_1.id
  vpnservice_id = openstack_vpnaas_service_v2.vpnaas_service_1.id
  psk = var.psk
  peer_address = var.fritzbox_wan_ip
  peer_id = var.fritzbox_wan_ip
  local_ep_group_id = openstack_vpnaas_endpoint_group_v2.vpnaas_endpoint_group_subnet_1.id
  peer_ep_group_id  = openstack_vpnaas_endpoint_group_v2.vpnaas_endpoint_group_cidr_1.id
}

data "openstack_networking_secgroup_v2" "networking_secgroup_1" {
  name = "${var.os_user}-sec"
}

resource "openstack_networking_secgroup_rule_v2" "networking_secgroup_rule_1" {
  direction = "ingress"
  ethertype = "IPv4"
  protocol = "tcp"
  port_range_min = 22
  port_range_max = 22
  remote_ip_prefix = var.fritzbox_cidr
  security_group_id = data.openstack_networking_secgroup_v2.networking_secgroup_1.id
}

output "vpnaas_service_external_v4_ip" {
  value = openstack_vpnaas_service_v2.vpnaas_service_1.external_v4_ip
}

Configure connnection

terraform 0.13upgrade -yes .
terraform init
terraform plan
terraform apply -auto-approve

Fix / import already existing security group rule

SECURITY_GROUP_ROLE_ID=$(openstack security group rule list --ethertype ingress --protocol tcp -f json | jq -r '.[] | select(."IP Range" == "192.168.178.0/24") .ID')
openstack_networking_secgroup_rule_v2.networking_secgroup_rule_1 ${SECURITY_GROUP_ROLE_ID}

Links
https://registry.terraform.io/providers/terraform-provider-openstack/openstack/latest/docs/resources/vpnaas_site_connection_v2
https://registry.terraform.io/providers/terraform-provider-openstack/openstack/latest/docs/resources/networking_secgroup_rule_v2