https://www.reddit.com/r/Windows10/comments/9nlxk7/is_there_a_way_to_download_the_microsoft_photos/

Plug the web store URL into https://store.rg-adguard.net, get the correct .appxbundle from the generated list of direct download links, then open a Power Shell window in the folder where you downloaded the appxbundle and enter Add-AppxPackage .\exactfilenamehere.appxbundle - note that this assumes you do not have the App Installer app installed, which would allow you to just double-click the downloaded appxbundle and skip Power Shell.