If you are using a manual method of updating the firmware on your range extender, you do not require internet to access the dlinkap.local interface. You only require the Internet to download the firmware for your Dlink range extender. You can download a new firmware available for your range extender from the official support website of Dlink. Sometimes during the update process, the settings get to revert to the default settings.
https://dlink-dlinkrouterlocal.com