Although Mono In > Stereo Out inserts are not supported, you can use Dual Mode and this will allow you to accomplish the same thing with a different routing UI.

• Enable Dual Mode on the H90: https://cdn.eventideaudio.com/manuals/h90/1.10.7/content/edit-modes/routing.html#dual-routing
• Connect your guitar/instrument to H90 input 1
• Connect H90 output 1 to the mono input of your amp sim/effect pedal
• Connect the stereo outputs of the amp sim/effect pedal to H90 inputs 3-4
• Connect H90 outputs 3-4 to your amps/monitors.
• Use the Routing menu to determine where you’d like your H90 and external effects in the signal chain

This will allow you to run mono effects on the H90 before your insert, and stereo effects after the insert. The stereo/mono signals will all be handled correctly with this configuration.