Opening Instagram app on your phone, you can search for the video you want and tap on it.
If you see the picture or video you like, you can tap on the mark aside the account name, and choose the "copy link" option, then it will turn to InsTube app.
You can choose the format of JPG or MP4. Additionally this side will show you the network condition in order to remind you.
The button on the upper right corner will show you the download process and you can tap on it. Then you can see the downloaded video on your files list. Tap on the green play button to watch it.