The PLC storage temperature is rated at -20 to +85 degree C. If it is contained inside an enclosure that's all good.
In fact most of the components including the CPU, with the exception of 2 or 3 ICs on the PLC are rated below -20 degree C. Those components rated for > 0 degree temperature may or may not work when operating at lower than rated temperature. But they will not be damaged when stored at - 20 degree C.