How to disable SimpleHMD VR moble game distortion

I got a headache in VR mobile game about meshes horriblely distortion in game.

As you can see at the center of picture 1’s rectangle

but in the picture 2 . when I tried to rotated the divece to changed camera rotation to 45 degree the rectangle was distorted to square

how can I disable this distortion issue in VR mobile game