Part of mesh is invisible when importing FBX from Blender

[disss]

So my problem is when i'm importing my model from Blender, part of it becomes invisible.
Here's demo:
[in blender] ->

[on mixamo] ->

All I can assume is that it is related to Weight Paint in Blender. I need some help with it.

[Ussnorway]

put the arms in T pose, have a small gap between the legs and add a neck under the head will give much better results

p.s, yes the way the legs deform is partly because of the Weight Paint but its also because of your basic shape i.e, when the legs move there is nowhere for those vertices to go