This issue will occur when you restore your new iPhone or iPad from an iCloud Backup. All media is contained within the Camera Roll or Photos app, so photos and videos does not show up on the new iPhone or iPad. There are two reasons behind this issue i.e. "Disabled iCloud Backup for Camera Roll" and "Sometimes, it takes a lot of time for photos and videos to appear. If you disabled iCloud Backup for Camera Roll, then it ensures that the photos and videos are backed up too.
Disabled iCloud Backup for Camera Roll:
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