IPhone randomly saying invalid sim?

So I bought an iPhone 7 from someone I knew that was stolen. I paid 25 for it then spent 80 for a guy to jailbreak and remove the iCloud. He put something on the SIM card and told me not to take it out. The phone works perfectly but maybe after 2 hours it’ll say invalid sim then I have to restart and it’ll be back... show more So I bought an iPhone 7 from someone I knew that was stolen. I paid 25 for it then spent 80 for a guy to jailbreak and remove the iCloud. He put something on the SIM card and told me not to take it out. The phone works perfectly but maybe after 2 hours it’ll say invalid sim then I have to restart and it’ll be back to normal. What should I do and I don’t really have the money to get it checked out again.
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