Insert mode can be thought of as 1 main audio path for the H90 (I/O 1-2) , with 2 mono or 1 stereo insert points. These insert points can be placed anywhere in the main path.
Dual Mode instead has 2 paths (Path 1 = I/O 1-2, Path 2 = I/O 3-4). There are no insert points in Dual Mode.
It sounds like you want to be using Dual Mode for your setup. Let me know if that clears things up.