Hi there @erica30; thanks for joining our Community!
Assuming that the app and your device's OS are up to date, you should find an option to have video files from your iPhone's gallery saved to Dropbox through the camera uploads settings on the mobile app. Here are the steps:
1. Go to the "Home" tap
2. Tap on the gear icon in the upper-left corner
3. Select “Camera Uploads” and toggle the “Upload videos” switch to on.
I hope this helps, Erica. Please let me know if you have any more questions.