In the video you will see how you may be able to retarget Mannequin Animations to your mixamo rigged characters and why it causes trouble when you try to rig and it does not look good. You will also understand how you can add root bone to your characters inside of Unreal engine.
https://dev.epicgames.com/community/learning/tutorials/v3KZ/unreal-engine-retargeting-animations-from-mannequin-ue-to-mixamo-and-back-again
Thank you for sharing your insights with us. Your tutorial was very helpful in understanding the retarget process from Manny to Mixamo. However, the background music was a bit loud.