Well, now you are saying you want to do both: watch on iPad and watch on TV, That’s different procedures.
This point I’ll make next is an important one. The files on the WDTV drive are on Shared drives, and apps that can stream DLNA format movies look for devices that are Media servers. Only certain devices are Media servers, PCs are, my Pogoplug w/drives is, and so is my WD NAS where I store all my media files…
Since my favorite File Browser app only sees Shared drives, it cannot cast to the WDTV. It CAN cast a movie to a $30 Chromecast device. So, this is one solution that works.
IF you have a home network, you can also use the free app called Belkin MediaPlay in Apple app store. It explors your network and sees only servers (not shares) so I can find an mp4 movie saved on my NAS and “beam” it to the WDTV. This works well, too.
Either of the above solutions work if you have the appropriate equipment and/or a home network set up. Otherwise, just use the WDTV remote to view movies stored on its drive on your TV. That’s what I do, since using the apps is an unnecessary procedure anyway.