Simple Mesh Combine
Simple Mesh Combine – download Unity asset
One click mesh combine tool
Easily combine meshes by attaching this script to a GameObject containing meshes with the same material.
Click the Combine button to merge meshes.
To decombine just click the Release button.
Easily decrease draw calls to improve game performance.
More Info: _https://assetstore.unity.com/packages/tools/utilities/simple-mesh-combine-8748