Improving triangulation on AutoCAD-generated stl files
The phrase "triangulation of the file is rather rough" is somewhat vague, but one can interpret it to mean that the surface is what is considered "low poly" in the 3D modeling world.



From Thingiverse, this low poly fox shows an intentionally reduced poly surface. I'm not suggesting that your models appear this distorted, but it may give a hint to what the service is referencing.

Consider to load your model into a program such as Meshmixer, which will show you the triangles in 'W'ireframe mode. If there are few triangles over a surface, you can get the aforementioned effect.

Meshmixer also allows you to increase the mesh count, possibly improving the surface and satisfying the requirements of the printing service.