Is it possible to delete the old rootpart?
Pretty sure that will break the whole rig.
I believe that the issue originates from the fact that this character might not be compatible for Roblox. My advice is to try to replicate the mesh in Blender with just the body and armature, without any rootpart.
Read these two articles from the Roblox Creator Documentation:
After importing the new mesh to studio, add an invisible part in the middle of the character that serves as the HumanoidRootPart and a Head if the character is just one mesh and the limbs are not separated.