Magento – Shipping address custom attribute value not getting in RateRequest object in Carrier Model in magento2.x

I have added the drop-down custom field on checkout page with custom values.

InstallSchema.php

$connection->addColumn(
                $installer->getTable('quote_address'),
                'mob_type',
                [
                    'type' => \Magento\Framework\DB\Ddl\Table ::TYPE_TEXT,
                    'nullable' => true,
                    'default' => NULL,
                    'length' => 255,
                    'comment' => 'Mob Type'
                ]
            );
        $connection->addColumn(
                $installer->getTable('sales_order_address'),
                'mob_type',
                [
                    'type' => \Magento\Framework\DB\Ddl\Table ::TYPE_TEXT,
                    'nullable' => true,
                    'default' => NULL,
                    'length' => 255,
                    'comment' => 'Mob Type'
                ]
            );
        $installer->endSetup();

Plugin

use Magento\Checkout\Block\Checkout\LayoutProcessor;

class MobPlugin
{
    public function afterProcess(LayoutProcessor $subject, $jsLayout) {
        $customAttributeCode = 'mob_type';
        $customField = [
            'component' => 'Magento_Ui/js/form/element/select',
            'config' => [
                'customScope' => 'shippingAddress.custom_attributes',
                'template' => 'ui/form/field',
                'elementTmpl' => 'ui/form/element/select',
                'id' => 'drop-down',
            ],
            'dataScope' => 'shippingAddress.custom_attributes.mob_type',
            'label' => 'Mob Type',
            'provider' => 'checkoutProvider',
            'visible' => true,
            'validation' => ['required-entry' => true],
            'sortOrder' => 150,
            'id' => 'drop-down',
            'options' => [
                [
                    'value' => 'local',
                    'label' => 'Local',
                ],
                [
                    'value' => 'vip',
                    'label' => 'VIP',
                ]
            ]
        ];

        $jsLayout['components']['checkout']['children']['steps']['children']['shipping-step']['children']['shippingAddress']['children']['shipping-address-fieldset']['children'][$customAttributeCode] = $customField;

        return $jsLayout;
    }
}

etc/di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Checkout\Model\ShippingInformationManagement">
        <plugin name="save_custom_field" type="Namespace\CustomModule\Plugin\Checkout\SaveAddressInformation" />
    </type>

</config>

SaveAddressInformation.php

class SaveAddressInformation
{
    protected $quoteRepository;
    public function __construct(
        \Magento\Quote\Model\QuoteRepository $quoteRepository
    ) {
        $this->quoteRepository = $quoteRepository;
    }
    /**
     * @param \Magento\Checkout\Model\ShippingInformationManagement $subject
     * @param $cartId
     * @param \Magento\Checkout\Api\Data\ShippingInformationInterface $addressInformation
     */
    public function beforeSaveAddressInformation(
        \Magento\Checkout\Model\ShippingInformationManagement $subject,
        $cartId,
        \Magento\Checkout\Api\Data\ShippingInformationInterface $addressInformation
    ) {
        $shippingAddress = $addressInformation->getShippingAddress();
    $shippingAddressExtensionAttributes = $shippingAddress->getExtensionAttributes();
    if ($shippingAddressExtensionAttributes) {
        $customField = $shippingAddressExtensionAttributes->getMobType();
        $shippingAddress->setMobType($customField);
    }

    }
}

But once click on shipping rate of radio button Magento call guest or user API to carrier model, In carrier model all request information passed but my custom attribute values not getting.

-module-offline-shipping\Model\Carrier\Flaterate.php

private function getShippingPrice(RateRequest $request, $freeBoxes)
    {
       echo $request->getMobType(); exit; // empty result
        $shippingPrice = false;

        $configPrice = $this->getConfigData('price');
        if ($this->getConfigData('type') === 'O') {
            // per order
            $shippingPrice = $this->itemPriceCalculator->getShippingPricePerOrder($request, $configPrice, $freeBoxes);
        } elseif ($this->getConfigData('type') === 'I') {
            // per item
            $shippingPrice = $this->itemPriceCalculator->getShippingPricePerItem($request, $configPrice, $freeBoxes);
        }

        $shippingPrice = $this->getFinalPriceWithHandlingFee($shippingPrice);

        if ($shippingPrice !== false && (
                $request->getFreeShipping() === true || $request->getPackageQty() == $freeBoxes
            )
        ) {
            $shippingPrice = '0.00';
        }
        return $shippingPrice;
    }