Open the Shortcuts app on your iPhone and create a new shortcut.
Add a "Get Contents of URL" action and enter your generated URL (you can copy it from the form above).
Add a "Set Wallpaper" action, select the image from the previous step, and choose "Lock Screen" (disable perspective zoom for best results).
In the Automations tab, create a new automation to run this shortcut weekly (or daily/monthly depending on your calendar type) to keep your wallpaper updated automatically.