Maybe, you have solved your problem. If not yet, then use EDITED: app against forum rules on your device. With this app, you can download or watching videos from YouTube without facing any problem...
This app is not available in the App Store, and it’s also not available in the Google Play Store. The reason: it allows downloading videos, and neither Google nor Apple allow that, for copyright concern.
The only way you can properly download YouTube videos is if the person posting the video specifically allows it. If so, the option to download appears.