Magento 2 | Access custom order attribute

check if the column is created in sales_order table

Now getting the values

 $order->getMediabasebestellnummer();

Or

$order->getData('mediabasebestellnummer');

For adding the data

 $order->setMediabasebestellnummer('test value')->save();

or

 $order->setData('mediabasebestellnummer','test value')->save();

or

$data = array('mediabasebestellnummer'=>'test');
$order->setData($data)->save();`

Make sure you have file fieldset.xml in your module to convert data from quote to order.

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:DataObject/etc/fieldset.xsd">
    <scope id="global">
        <fieldset id="sales_convert_quote">
            <field name="mediabasebestellnummer">
                <aspect name="to_order" />
            </field>
        </fieldset>
    </scope>
</config>