How to merge generated meshes

Hi, I have a vision of a procedural tile based (I like the tile based aesthetic) terrain generation but I am having trouble understanding how mesh merging works. I will attach my code and hopefully someone can point me in the right direction (or even suggest a better approach to this problem).

I expected this code to produce a 5 x 5 grid of tiles at y coord 0, but I don't see anything when I run it. I think my problem lies in setting the transform in the CombineInstance. At the moment I am using Matrix4x4.TRS to produce a transform but to be honest I don't really understand what I am doing, and the documentation is pretty sparse. In fact if anybody knows of a good resource for learning these kinds of processes I would be very grateful.

Thanks for reading, and for any help

 public class TilesGenerate : MonoBehaviour
 {
 
     Mesh currentMesh;
     public float tileHeight = 0.2f;
 
     public int xWidth = 5;
     public int zDepth = 5;
 
     // Start is called before the first frame update
     void Start()
     {
         gameObject.AddComponent<MeshFilter>();
         gameObject.AddComponent<MeshRenderer>();
         currentMesh = GenerateMesh();
         GetComponent<MeshFilter>().sharedMesh = currentMesh;
     }
 
     // Update is called once per frame
     void Update()
     {
         
     }
 
     Mesh CreateTile(float x, float z)
     {
         Mesh mesh = new Mesh();
         Vector3[] Vertices = new Vector3[] {
             //top quad
             new Vector3(x + 0, 0, z + 0), //0
             new Vector3(x + 1, 0, z + 0), //1
             new Vector3(x + 1, 0, z + 1), //2
             new Vector3(x + 0, 0, z + 1), //3
 
             //bottom quad
             new Vector3(x + 0, -tileHeight, z + 0),     //4
             new Vector3(x + 1, -tileHeight, z + 0),     //5
             new Vector3(x + 1, -tileHeight, z + 1),     //6
             new Vector3(x + 0, -tileHeight, z + 1),     //7
 
 
             //top quad copy for sides
             new Vector3(x + 0, 0, z + 0),  //8
             new Vector3(x + 1, 0, z + 0),  //9
             new Vector3(x + 1, 0, z + 1),  //10
             new Vector3(x + 0, 0, z + 1),  //11
 
             //bottom quad copy for sides
             new Vector3(x + 0, -tileHeight, z + 0), //12
             new Vector3(x + 1, -tileHeight, z + 0), //13
             new Vector3(x + 1, -tileHeight, z + 1), //14
             new Vector3(x + 0, -tileHeight, z + 1)  //15
 
             
         };
 
         int[] Triangles = new int[] {
             //top quad
             2, 1, 0,
             3, 2, 0,
 
             //bottom quad
             4, 5, 6,
             4, 6, 7,
 
             //sides
             12, 15, 11,
             12, 11, 8,
 
             12, 8, 9,
             9, 13, 12,
 
             10, 14, 13,
             13, 9, 10, 
 
             15, 14, 10, 
             10, 11, 15
         };
 
         mesh.vertices = Vertices;
         mesh.triangles = Triangles;
         mesh.RecalculateNormals();
 
         return mesh;
     }
 
     Mesh GenerateMesh() {
         CombineInstance[] combine = new CombineInstance[xWidth * zDepth];
         int i = 0;
         for (int z = 0; z < zDepth; z++)
         {
             for (int x = 0; x < xWidth; x++)
             {
                 Mesh tile = CreateTile(x, z);
                 combine[i].mesh = tile;
                 combine[i].transform = Matrix4x4.TRS(new Vector3(x, 0, z), Quaternion.identity, Vector3.one);
                 i++;   
             }
         }
 
         Mesh mesh = new Mesh();
         mesh.CombineMeshes(combine);
 
         return mesh;
 
 
         
     }
 
     void OnDrawGizmos()
     {
         if (currentMesh != null)
         {
             foreach (Vector3 vertex in currentMesh.vertices)
             {
                 Gizmos.DrawSphere(vertex, 0.15f);
             }
         }
     }
 }