No connection, cannot ring people up

Hello.
I am enquiring on behalf of a friend of mine who bought a Samsung cellphone from an unreliable shop and paid half the price of the new one.
Now he has the Wifi symbol but not the cellular connection strength symbol.
The sim card is good and has money (credit) in it.
My friend can see the list of contacts and calls but cannot connect with anyone
What can be the reason?
Thanks a lot and best regards

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