Hi,
Despite the special .fbx download is not supported anymore, you can still use Mixamo! You only have to make sure that before you add your animation to the “Download Queue” you tick the “In Place” checkbox while previewing the animation. This way your skeleton should be properly setup for using RootMotion in UE4 (I can’t remember but you might don’t even need to activate RootMotion in UE, it might work ‘out of the box’).