What you’ve tried so far would be helpful.

The first essentials:

• 5-pin DIN cable from MIDI Mate MIDI Out to H90 MIDI In
• MIDI Mate & H90 set to the same MIDI Channel
• If I recall correctly, they should both start out at Channel 1 as the default.

Starting Point Links:

https://cdn.eventideaudio.com/manuals/h90/1.7.5/content/system-menu/midi.html

https://www.rocktronusa.com/uploads/2/2/2/5/22258814/midimate.pdf

Once confirmed, we’ll move on to Program / Bank changes & HotSwitch mapping to CC messages.