You don’t need to trigger the material switch directly inside Matinee. Just set it up in your level blueprint to play the matinee on begin play or through a trigger and then add a delay to match the time you want the lights to turn on. For example if they’re supposed to turn on at 10 seconds into your sequence, then set the delay to 10 seconds and then trigger the EMISSIVE (not massive) switch in your level blueprint immediately after the 10 second delay.
Its just a matter of triggering things in the blueprint at the correct time.