Thank you for your reply. Unfortunately we can't import the fbx files into unreal engine, it doesn't allow us to do so. So we're already blocked at import time, so it's impossible to modify them inside the software.
When you import them (even the T pose) you get the error message: "Error Mesh contains Hips bone as root but animation doesn't contain the root track".
I spent a good 2 hours trying to debug the problem online, but I ended up with videos that show us in summary how to import mixamo animations to unreal engine, and each time they already have a reference model for the animations.
That's why I'm quite disappointed, I'd seen a lot of comments about Unreal engine, but I don't know if they've really succeeded in importing the animations.