Masking 3D object with custom shader in Unity - unity3d

Question

I'm making a 2D mobile game but I want to use 3D objects in shop section. So, in the shop section, I have a scroll rect and a viewport. I can scroll and the mask component works as expected with 2D objects. But when I add 3D objects, the objects getting out of the mask. I researched the forums and I find a custom shader. When I apply the shader to the objects materials, it partially works. But the depth or shadows are not working properly. I know not much about shaders. You can take a look at this Image. Box on the right is the original one, box on the left is the one with custom shader.
My shader code is here;
Shader "Custom/NewSurfaceShader" {
Properties {
_Color ("Color", Color) = (1,1,1,1)
_MainTex ("Albedo (RGB)", 2D) = "white" {}
_Glossiness ("Smoothness", Range(0,1)) = 0.5
_Metallic ("Metallic", Range(0,1)) = 0.0
_StencilComp("Stencil Comparison", Float) = 8
_Stencil("Stencil ID", Float) = 0
_StencilOp("Stencil Operation", Float) = 0
_StencilWriteMask("Stencil Write Mask", Float) = 255
_StencilReadMask("Stencil Read Mask", Float) = 255
_ColorMask("Color Mask", Float) = 15
[Toggle(UNITY_UI_ALPHACLIP)] _UseUIAlphaClip("Use Alpha Clip", Float) = 0
}
SubShader
{
Tags { "RenderType"="UI""Queue"="Transparent" }
LOD 200
ZTest[unity_GUIZTestMode]
Blend SrcAlpha OneMinusSrcAlpha
ColorMask[_ColorMask]
Stencil
{
Ref 1
Comp LEqual
Pass Keep
ReadMask [_StencilReadMask]
WriteMask [_StencilWriteMask]
}
CGPROGRAM
#pragma surface surf Standard vertex:vert fullforwardshadows
#pragma target 3.0
#include "UnityUI.cginc"
struct Input
{
float2 uv_MainTex;
float4 worldPosition;
};
void vert(inout appdata_full v, out Input o)
{
UNITY_INITIALIZE_OUTPUT(Input, o);
o.worldPosition = v.vertex;
}
sampler2D _MainTex;
half _Glossiness;
half _Metallic;
fixed4 _Color;
float4 _ClipRect;
void surf (Input IN, inout SurfaceOutputStandard o) {
// Albedo comes from a texture tinted by color
fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
o.Albedo = c.rgb;
// Metallic and smoothness come from slider variables
o.Metallic = _Metallic;
o.Smoothness = _Glossiness;
o.Alpha = c.a;
o.Alpha *= UnityGet2DClipping(IN.worldPosition.xy, _ClipRect);
#ifdef UNITY_UI_ALPHACLIP
clip(o.Alpha - 0.001);
#endif
}
ENDCG
}
FallBack "Diffuse"
}