Go to Smart Search & Instant Search admin panel → Integrations page, and turn the toggle On. Next, click Apply Changes.
After the changes applied, you need to click force re-indexation. The indexation with active Infinite Options integration usually takes more time than usual.
Note: You should run force re-indexation each time you changed the custom options sets. Wait approximately 5 minutes after the changes saved, go to Smart Search & Instant Search dashboard and start indexation.
After the indexation completed, you will see the filters by Infinite Options custom sets in the Search Results widget.