Street Twin Overheating

I've also got a 2017 ST but never had that problem and the fan does come on when things heat up. Perhaps you need to buy the relevant Haynes, which will give you the circuit diagram(s) you're looking for.

 

The relay is behind the RHS side panel. I assume you've checked the fuse (4) and it's not blown.

 

In my opinion, the alarm light should light up before the engine overheats and stalls.
It is difficult to explain how you had no alarm.

 

If the high temp warning light isn't coming on and the fan isn't cutting in are you certain the bike is actually overheating or maybe is something else causing the hot stalling/starting issue? If the bike does overheat I would expect coolant to be blown out of the expansion tank.

 

I agree with pegscraper.