Thank you AnemicRoyalty!
While looking for the function to flip faces in Gmax, I came across the 'Clone' function which I reckon I should have used instead of 'Mirror', because that's what I did now which solved the problem.
I don't think I should ask why or how I caused the normals to be inverted, hmm.
Wouldn't the TwoSided material use a little more processing power?
@ TKF, I really should have updated the screenshots with proper texture and material edits... because black really makes it look like a mobile bomb indeed!