So, I had this issue with heterogenous volumes outside of the frustum. Bound scale doesnt work with it, as it does with static meshes. So, we can make the heterogenous volume a child of an object with a larger bound, and use that parents bounds via the checkbox on the heterogenous volume called Use Parent Bound (something like that). Then make the parent invisible, and the child heterogenous volume should have a new bounds that can get picked up by the camera frustum. You might need a combination of overscan on the camera as well.