No video mode with camera in iOS 13.3
I recently updated to iOS 13.3 and found out that there is no option to take a video in the camera app. Is iPhone 6s Plus too old?
iPhone 6s Plus, iOS 13
Yes, it is old, but not old enough to prevent you from taking a video with your camera.
Have you already tried a forced restart of your 6s+? :--> Force restart iPhone – Apple Support
Regards
Giulio