the larger pop-up window that lets you scroll through the photos
That specific functionality as described isn’t in WordPress as-is, but many plugins and some themes provide scrolling lightboxes, like Jetpack: https://jetpack.com/support/carousel/
WordPress very recently added a lightbox, but it doesn’t have scroll functionality. You click the image, the light box opens, you close it, and you click another.
What plugins do you have installed?
Aksimet, Jetpack Boost, LifeSpeed, PHP Compatibility Checker, SEO Press, Smash Balloon.
What’s weird is that it worked great before. “Before” might include before migrating from iPower to Namecheap hosting. I’m just noticing the problem now.
I’ll try to set up the Jetpack Carousel as shown on the link you sent.
Thanks!