Pictures don't "scroll" correctly in a Gallery

I apologize if this has already been asked and answered. If it has, please show me a link and I’ll go away quietly. I’m also an idiot with this stuff, so I’ll just come clean with that right up front!

My website (www.toonturning.com) has multiple pages, with a gallery of pictures on each page. When looking at the pictures on the “outer page”, the gallery looks fine. When you click on one of the pictures, a larger image is supposed to display, along with “next” and “previous” buttons to scroll through the images in the gallery. What happens, though, is the next and previous buttons just seem to link to other random pictures from the media library, that may or may not even be in that page’s gallery. I’ve also seen it link to pictures that are in the media library, but aren’t published on any particular page.

I tried deleting the galleries and just adding the pictures on their own, one at a time, not using a gallery, but if you open one of the pictures, there are still the “next” and “previous” buttons that link to random pictures, so the problem is still there.

I tried deleting every single image in the media library, then adding back the pictures for a gallery all together at once as I built the gallery, but the problem is still there.

I tried reinstalling the latest version of WordPress and disabling the few plugins that are running, but the problem is still there.

There is no doubt in my mind that user error is likely the root cause of the issue. Can anyone help me figure out what I’m doing wrong?

Please and thanks!