Fixing white portion of a merged TIFF

I am trying to add a merge layer to my project. I noticed that my merged layer has a region that is way too white, I am trying to fix that white space by adjusting the interval value from 0-255 to 0-1250, but whenever I set the values, it goes back to 0 – 255.

Here’s a picture of my merged map, the link for the TIFF files are in here

Basically what I want to do is see what’s in that white space, by increasing the interval to 0-1250, according to the original map scale. From what it looks like to me, it appears to not have any texture in that white area, therefore I would like to confirm it, by changing the interval.