Trick on How to Download Private Videos upload in Instagram
Step 1: Go find and download the Xposed installer. (If you don't have any idea about Xposed installer you may find my previous thread about Xposed installer and how to install it)
Step 2: After downloading and installing Xposed installer go find Xinsta in the downloads corner in Xposed installer app.
Step 3: Download and install again Xinsta then marked it as checked to be active then reboot your device.
Step 4: Lastly, you can modify its settings to download pics and vids by single tap double or long press and etc. or leave it as is. Then now you can download pics and vids in Instagram freely.