The video files need to be coded with an iTunes video encryption if you are downloading the videos on any iOS device.

Option 2:
Download the videos on to your computer and move them into a Google drive folder.
While your iOS device is connected to the internet, open your google drive and find the videos you wish to have access to offline.
The video image will appear as a screenshot of the video with name, file type and two small icons in the bottom right corner of the box.
The small icon farthest to the right is an info button (looks like a lower case "i" in a circle).
Select that info button and a details box will appear.
You can then switch keep on device to on from off and the video will be able to be accessed with no internet connection.

Option 1