Photos is a lousy choice to watch movies. Yes, it would work, but only with the bare minimum of features and controls. One of the other apps I mentioned would be better.
Search for
Wi-Fi storage for iPad on
Amazon. You'll see several options. You will note that some of them are hard drives, but they have a wi-fi connection that will work with your iPad. Some of them will let you play media directly from the storage. All of them require a companion app (usually free).
These are the devices I mentioned before. I can't recomend any of them, since I don't use any of them. I do recommend that you look for reviews on the companion apps as well as the hardware, since the companion app will be central to the experience.
Note: You can connect to either the storage device or internet, not both at the same time. Keep this limitation in mind.