Try Standard SKU and Static allocation method

operations/template/vpn.tf

@@ -3,8 +3,8 @@ resource "azurerm_public_ip" "vpn" {
   location            = data.azurerm_resource_group.group.location
   resource_group_name = data.azurerm_resource_group.group.name
 
-  allocation_method = "Dynamic"
-  sku               = "Basic"
+  allocation_method = "Static"
+  sku               = "Standard"
   #   below tags are managed by CDC
   lifecycle {
     ignore_changes = [