I am an absolute beginner with using primitives in Cesium and tried constructing a 3D arc which worked
nicely using PolylineVolumeGeometry. However, my arc is on the surface now and I would like to rotate it 90 degrees. I tried setting the modelMatrix of the geometry but I am doing something wrong with the transformation (as the geometry is rotated around the center of the earth).

I am clearly doing something wrong here and would be immensely grateful for some help to get me off the ground!

We’re looking to do the same thing. Have you found a solution?
Here is a sandcastle with some experiments to find a way to apply a model matrix to a PolylineVolumeGeometry through a GeometryInstance as defined in the documentation.
Unfortunately, this works for other geometries but not for this one.
Every time you want to apply a model matrix other than Cesium.Matrix4.IDENTITY, the geometry disappears.

In the sandcastle, you can play with a few commented lines to test different matrices and see the result.
Are we doing something wrong, or is it simply impossible to apply a model matrix to a PolylineVolumeGeometry?