How do poiklotherms survive at different temperatures even though enzymes work at specific temperature?
Instead of viewing poikilotherms as having the same temperature as the environment, view them as lacking an internal thermoregulatory system. Poikilotherms indeed suffer from not having the ability to regulate their internal temperature.

Homeothermy is ecologically advantageous, allowing optimal functioning at a range of temperature. Poikilotherms, therefore, are more restricted in their habitat range, have to depend on external mechanisms for maintaining temperature (eg: Hibernation, Shade-loving lizards, burrowing in desert animals) or simply die due to environmental change (eg: poikilothermic fish are the earliest to die from a thermal runoff into a water body).