If the reaction was endothermic, I’d expect the reverse to be exo - I.e. harder in hot temperatures.
Could you explain more? Would it be more difficult to get the reaction at higher temperatures? Also, would the ammonium nitrate break down in solution? I recall some high school chemistry where heating ammonium nitrate produces nitrous oxide.
If you know chemistry, it’s Le Chatelier’s principle. In particular, for a change in temperature: Le Chatelier's principle - Wikipedia
In brief, a system opposes any changes you make.
Or you could think of a chemical cold pack like a pack of ice. Its use is to absorb heat. So to recharge, you should do the opposite - take away heat. It’s going to be harder to take away heat using a hot surrounding than a cold surrounding.
You should rent The Mosquito Coast, ice making and pre-industrial survival in one
Buying bag ice at the gas station/convenience store/bait shop is a time-honored tradition!
~VOW