If the H90 is set to receive MIDI clock, but there is no MIDI clock being sent to the input, the looper transport will not function.

Please try going to the H90 tempo menu (press and hold Presets + Parameters) and make sure the Source is set to MIDI Clock.

If you stay on this menu, when you send MIDI clock to the H90 you should see the Tempo parameter update. If you change the BPM of the MIDI clock you are sending, you should see the H90 Tempo follow the BPM.