check your normals. In the top right of your viewport there is a dropdown that looks like 2 spheres overlapping with martching ants where they intersect, that is your viewport overlays, click that and select face orientation, then select all your meshes, just the meshes, tab into edit mode, and select all the faces and press shift+n. This will recalculate the normals. Anything blue is the “front” if you see it red, then it will appear transparent in most OGL viewers. I think Mixamo uses webgl so this is a problem you could have. if anything is still red, go one by one on the meshes that are inverted and hover over them, click L on the keyboard to sellect all connected parts, and do shift+n again. If it will not flip then in the lower left after recalculating the normals you will see a panel for recalculating normals, it may start minimized, pop that open and select “inside.” so it manually flips the normal.
Give that a try and see if that works. I would bet the whole front of your character is red.