Try this:

<admin>
    <routers>
        <advancedstock> <!-- put there the router key of the original module -->
            <args>
                <modules>
                    <Vendor_AdvancedStock before="MDN_AdvancedStock">Vendor_AdvancedStock_Adminhtml</Vendor_AdvancedStock>
                </modules>
            </args>
        </advancedstock>
    </routers>
</admin>

Then place your controller inside Vendor/AdvancedStock/controllers/Adminhtml and make sure it's called the same as the original controller.

Try this:

<admin>
    <routers>
        <advancedstock> <!-- put there the router key of the original module -->
            <args>
                <modules>
                    <Vendor_AdvancedStock before="MDN_AdvancedStock">Vendor_AdvancedStock</Vendor_AdvancedStock>
                </modules>
            </args>
        </advancedstock>
    </routers>
</admin>

Then place your controller inside Vendor/AdvancedStock/controllers and make sure it's called the same as the original controller.