The Elements App by Theodore Gray

I purchased this app and had it loaded on my iPhone 5S and 7, but I can't even find it in my iTunes purchased history so that I can update it. One message I get is that it is not available for OS 11, although I have updated to OS 12.4.1. for both the 5S and 7. And the other message I get is that it is no longer available in the app store; yet a search does come up for iPhone X updates.

How do I get this app updated and then loaded onto my new iPhone XS without having the buy it again? I saw that someone else had a similar app problem in which their purchased app had disappeared from their iTunes Purchased List and could only be found as an purchasable item.

Does anyone have any advice on how to get my purchased app working again and updated?