I would suggest connecting to the H90 in the following way if you want to use Dual Mode for send/return paths: https://cdn.eventideaudio.com/manuals/h90/1.9.7/content/edit-modes/routing.html#dual-routing

• Enable dual mode on H90 and configure the routing to be in pre/post mode: https://cdn.eventideaudio.com/manuals/h90/1.9.7/_images/Routing-Dual-Pre-Post.png
• Connect mixer Aux 1 (mono) to H90 input 1
• Connect mixer Aux 2 (mono) to H90 input 3
• Connect H90 output 1-2 (stereo) to stereo return 1 on mixer.
• Connect H90 output 3-4 (stereo) to stereo return 2 on mixer.

This should resolve your issue.