you've discovered the culprit

this is not a whack at you but illustrates how using correct terms and full and complete information can expedite solutions

yes, it could be the starter or the cable from the battery

cranking but not starting is more descriptive as opposed to "nothing happens" unless of course "nothing happens" included "it does not crank at all"

no starter engagement, no clicking

If I had to guess, the starter turned slower when it was hot, is that correct? Adding the 2nd battery gave it the extra jolt and away you went

good ole fahion basic stuff like push starting revealed that you had fuel/spark/air

 

might try a heat shield on the starter.

 

It doesn't make a sound no click won't turn over or anything

 

have you cleaned the battery cable ends. sometimes they can get a layer of carbon on them that won't let the power through. scrape with a knife or if side cable wire brush the heck outa it. sounds like your going to have to start searching with a volt meter and see if you can find where the power stops. good place to start would be the ignition wire on the solenoid, make sure your getting 12 volts when the key is turned to the start position.