The big problem is, that Photos iOS cannot show the metadata. And so it does not have any support to add the metadata. This is really disappointing on a device called "iPad Pro".
Yu could get yourself an app that can add metadata to Photos. I am using "ViewExif" on my iPhone, and I think you can also get it on a newer iPad. ViewExif can show you the keywords and captions added on a Mac, and you can also use it to add keywords and captions. Photos will still not show them in Photos on the iPad, but you can search for them in Photos and they will show in a search.
https://itunes.apple.com/us/app/viewexif/id945320815?mt=8
Metapho can also show Exif tags and edit them