I want to change a mesh material with a button

Hi everyone,
I’m trying to change a mesh material by clicking on a button. So I have three buttons on a HUD that are setting an index number for a material array when clicked, although it changes the material, it only shows me the default squared material. I’ve tried to plug a simple variable containing a material in the set material, but I’ve had no better results . I’m trying to make it work for mobile. it works fine with the shader model 5.So i’m beginning to think the set material is not optimised for mobile ?
Here is the object’s blueprint. Every comments would be most welcome.
Thank !!

Here is a little update. The set Material works fine with a static mesh when the lighting is not build, but as soosn as lightmaps are applied, the default material is applied, so I guess I shoul have to find a bit more complex way to assign my new material fot it to keep the lightmaps and the reflections…
All help will be welcome !!

Cheers