How do I regain access to a locked MacBook Air that’s not on Wi-Fi?

Hi. Im getting incredibly frustrated. I have taken back my MacBook Air from an employee who has left the business. I have no access to it, when I've managed to reset the password online I will receieve a 6 figure code to the MacBook, but it isn't connected to wi-fi as it's changed. so how can I get this code to the MacBook? Can I join a network from the safe start screen....?

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