Sharp language ahead, but i was infuriated at getting openWRT to work before this. Go away if you're easily offended.
I have mirrored the version I've been using for this guide. Though i encourage you to download the latest one.
Mirror: https://drive.google.com/open?id=1u--Ynws2FTW4X1oycYXp2xOZ4B8pvRt6