How to open backed up Photos library of external drive on iPad or iPhone?

Photos stored in external or cloud-storage will not be accessible using the native Photos App - unless the images are imported to the iPad's (or iPhone's) Photo Library.

Assuming that you are connecting the external drive (via a suitable USB Adapter as necessary) to your iPad, import of photos and videos is possible - subject to a couple of caveats:

• the SD Card must be correctly formatted as FAT, FAT32 or exFAT (FAT64)
• stored photos are stored within the expected DCIM folder

Start here:

Connect external storage devices to iPad - Apple Support

Import photos using Apple camera adapters - Apple Support

If you are unable to use the direct photo import feature, you can still manually access and copy photos using the iPad’s built-in Files App. To do so, just open the Files App - and in the Sidebar you will see your SD Card in the list of Locations. If necessary, open the sidebar by tapping the sidebar icon at top-left of the Files App Window. Photos can be copied to the iPad Photo Library, or copied to a folder of your choice either on your iPad or elsewhere.