If the phone was from Sky then the sim should work in it as all mobiles from Sky are unlocked so it might be that the one you have has developed a fault so it might be worth getting in touch with Sky to ask them to send you out a replacement which should be with you within 2-3 days via Royal Mail:
https://www.sky.com/help/articles/replacement-sim-sky-mobile
Select - No, I still need help >No, I want to get in touch > tick both boxes > Get in touch.
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