Shader question for ios - not working on the device

Hello, I'm building a POC to build a terrain with a surface shader I get from a tutorial. the shader is working fine in the editor but it is not correct on the device on in the simulator. I'm lost with the configuration and I would like to understand what's wrong with this shader.

 Shader "Custom/Terrain"
 {
     Properties {
         _Color ("Color", Color) = (1,1,1,1)
         _MainTex ("Terrain Texture Array", 2DArray) = "white" {}
         _GridTex ("Grid Texture", 2D) = "white" {}
         _Glossiness ("Smoothness", Range(0,1)) = 0.5
         _Specular ("Specular", Color) = (0.2, 0.2, 0.2)
         _BackgroundColor ("Background Color", Color) = (0,0,0)
         [Toggle(SHOW_MAP_DATA)]_ShowMapData ("Show Map Data", Float) = 0
     }
     SubShader {
         Tags { "RenderType"="Opaque" }
         LOD 200
         
         CGPROGRAM
         #pragma surface surf StandardSpecular  vertex:vert
         #pragma target 3.5
 
         #pragma multi_compile _ GRID_ON
         #pragma multi_compile _ HEX_MAP_EDIT_MODE
 
         #pragma shader_feature SHOW_MAP_DATA
 
         #include "../HexMetrics.cginc"
         #include "../HexCellData.cginc"
 
         UNITY_DECLARE_TEX2DARRAY(_MainTex);
 
         sampler2D _GridTex;
 
         half _Glossiness;
         fixed3 _Specular;
         fixed4 _Color;
         half3 _BackgroundColor;
 
         struct Input {
             float4 color : COLOR;
             float3 worldPos;
             float3 terrain;
             float4 visibility;
 
             #if defined(SHOW_MAP_DATA)
                 float mapData;
             #endif
         };
 
         void vert (inout appdata_full v, out Input data) {
             UNITY_INITIALIZE_OUTPUT(Input, data);
 
             float4 cell0 = GetCellData(v, 0);
             float4 cell1 = GetCellData(v, 1);
             float4 cell2 = GetCellData(v, 2);
 
             data.terrain.x = cell0.w;
             data.terrain.y = cell1.w;
             data.terrain.z = cell2.w;
 
             data.visibility.x = cell0.x;
             data.visibility.y = cell1.x;
             data.visibility.z = cell2.x;
             data.visibility.xyz = lerp(0.25, 1, data.visibility.xyz);
             data.visibility.w =
                 cell0.y * v.color.x + cell1.y * v.color.y + cell2.y * v.color.z;
 
             #if defined(SHOW_MAP_DATA)
                 data.mapData = cell0.z * v.color.x + cell1.z * v.color.y +
                     cell2.z * v.color.z;
             #endif
         }
 
         float4 GetTerrainColor (Input IN, int index) {
             float3 uvw = float3(
                 IN.worldPos.xz * (2 * TILING_SCALE),
                 IN.terrain[index]
             );
             float4 c = UNITY_SAMPLE_TEX2DARRAY(_MainTex, uvw);
             return c * (IN.color[index] * IN.visibility[index]);
         }
 
         void surf (Input IN, inout SurfaceOutputStandardSpecular o) {
             fixed4 c =
                 GetTerrainColor(IN, 0) +
                 GetTerrainColor(IN, 1) +
                 GetTerrainColor(IN, 2);
 
             fixed4 grid = 1;
             #if defined(GRID_ON)
                 float2 gridUV = IN.worldPos.xz;
                 gridUV.x *= 1 / (4 * 8.66025404);
                 gridUV.y *= 1 / (2 * 15.0);
                 grid = tex2D(_GridTex, gridUV);
             #endif
 
             float explored = IN.visibility.w;
             o.Albedo = c.rgb * grid * _Color * explored;
             #if defined(SHOW_MAP_DATA)
                 o.Albedo = IN.mapData * grid;
             #endif
             o.Specular = _Specular * explored;
             o.Smoothness = _Glossiness;
             o.Occlusion = explored;
             o.Emission = _BackgroundColor * (1 -  explored);
             o.Alpha = c.a;
         }
         ENDCG
     }
     FallBack "Diffuse"
 }