Glad to hear you figured it out.
The I/O behavior is described here: https://cdn.eventideaudio.com/manuals/h90/1.8.6/content/system-menu/io.html
When using Insert mode, I/O 1-2 will be the main path, I/O 3 is insert 1, and I/O 4 is insert 2. I/O 3/4 can be combined to be a single stereo insert.
