Frozen Sim Card Holder Failure
Well, I doubt the SIM itself is rated for -20°C unless it's really well made. It could also be a case that the sim being a low power device simply didn't ``compute'' correctly at such a low temperature, or gave insufficient signal level or shape.
If the card holder were effectively rated for -20°C its finger would have been designed to keep the contact in these condition, probably. Can you contact the manufacturer?
By design the sim holder contacts are self-cleaning since they 'brush' during mating, typically they are gold plated copper berillium. Also oxidation at such a low temperature is quite difficult, for how much it was subjected?
Check the ratings, they often don't specify for how much time it can survive