It looks like the background behind the feature section is too long, what kind of css do you have for the height?
I can't find any trace of css grid, but there is quite a bit of flex. Last time I looked, IE11 wasn't ok with all kind of flex (but it's a while ago since I looked).
You have a lot of out-of-view. Try to remove some of it (make a copy of the site to experiment on), especially from the feature section. If that helps - or if you find anything that helps - you may have to create a fallback without it for IE, and add support for flex, just as we have been doing with grid.
I could take a look for you, but I would need your project for that. Not everything, perhaps, if you make a copy first, you could delete all the other pages from the copy and just send me the index page (including dependency files).