If the photo or video was not on the phone at the time it was backed up, it would not be included in the backup. However, if this only happened recently, you may have an older backup in iCloud that would contain these that you could restore to. You would have to restore the entire backup, not just photos or videos.
Restoring to an iCloud backup has to be done in the initial setup process. If you have any data on your phone now that is not in the backup that you want to save you'll need to do that first. Then go to Settings>General>Reset and tap Erase All Content and Settings. You will then go through the setup screens as you did when the phone was new. When given the option, choose Restore from iCloud Backup (see image below). Be sure you are connected to wifi and your charger as this will take some time to finish.