RenderTexture - NGUI Texture on android render problem

Hi guys,
I don't know does this is suitable place to post this thread or should move it to somewhere. If this's the wrong place please let me know.

Currently I have to working with render texture to render a mini map in my game scene. This is my env:
* Unity 5.6.3p1
* NGUI 3.11.4
* Test device is xperia Z with android 4.2.2.

Here is my problem. I use render texture to draw my map to an UIImage Object. This object locate in 3D space with second camera, the content this object show is what happen in main camera. Everything work well on editor until I build to android, I can not see my map display, It's not black or grey or somethings else, just transparent - I'm not sure (because I can see the background behind this object so I think so). And I have googled almost 3days, finally I have to wave the white flag... and this is my work around, I just create a 3D cube, a material from that renderTexture use the Unlit shader. And finally it's... still not work (as expect).

I hope I can find out some one who got the same problem, if you know the reason why and how to fix it by a right way, please show me or we can discuss about this problem.

Sorry for my broken english. Thanks to read my thread
Vince