After few days on and off modeling and rigging, I had extra 2 days fixing the issue 3D Studio Max always have with mirroring geometry: inside 3DS, all good, but once you export it, normals are flipped!
Manually flipping normal won’t fix it.
Add Normal modifier, won’t fix it either.
What worked for me in Skechfab, as to: create new geometry like sphere, convert to editable poly, attach to the fliped/mirroed geometry, and then delete the sphere.
I hope this might bring comfort to some poor soul dealing with this.
Max version is 2019.
Here the final result of my Digit2 robot: