Landscape is invisible in places

I’m making a landscape, and even though I’m using “low-poly” trees, my NVIDIA GeForce 980 Ti turns red and I can hear the fan working hard. I thought it was because maybe it was trying to display too much at once, so I read some stuff online that suggested streaming your level. I tried a bunch of stuff to try to reduce the load on the GPU, and I somehow messed something up. The landscape disappears a certain way from the character, but in places, it’s invisible even when I’m up close to it, and even sometimes on top of it. Here are some screenshots to show what I mean. I’m using UE 4.25.4 on Win10. I’d be happy to provide any other info if you want.

If someone can help me return it to normal, I’d really appreciate it. Thanks in advance.