This question already has an answer here:
I used to keep all of my photos (literally 40K or so) on my iPhone. I did eventually fall into the iCloud trap, and have still not figured out a way to assure that a healthy number of my full resolution photos will be available to me when I am on a plane: the one time I know I'll have 5 hours with nothing to do but edit photos from the last decade. I know that "recent" full resolution photos are stored on the phone, but going back to what date? And how does that help if I choose a picture from 2014 to edit? Thoughts about how to edit while in "airplane mode?"