AGK's Object Loader is frustratingly basic... especially if you're coming from Dark BASIC Professional.
As I recall, when you load an object it will treat it as a Single Mesh, Static Object.
What's more is it will ALWAYS load as FVF338 (Position, Normal, UVSet0)
It won't even load the Vertex Diffuse Colour., not until you change the Mesh Format.
Animation is also handled as a separate (and in an archaic DAE Format)... honestly I think 3D Support was more or less an after thought., with the bare minimum of DBP Features being ported over.
In any case., a good practice to get into is to create a blank object with the correct FVF Format (which yeah it still uses identical to DBP / DirectX 9 despite being OpenGL)., then load individual Mesh as attached Object Mesh; then you can individually access and change things.
If you want to use a "Single Mesh" then just rearrange the UV Map., to use a Single Texture.
What the last number of the "SetObjectMeshImage" is., is essentially the Texture Stage used in Shaders... like DBP these come with pre-defined roles., but you can change these within a Shader.