Welcometo AJOR mate
Sounds like your secondary fan is a bit lazy! it should turn on with the air con if it works.
But yyou are right, it's not kicking in right...
It comes down to CFM (Cubic Feet per Minute) your airflow, the stock electric fan draws 1500 CFM, your aftermarket fan should draw
At Least! 3200 CFM to keep the jeep cool