iPhone locked to cellular service

Only the carrier to whom the phone is locked can remove the lock. You need to get the seller to contact the carrier about unlocking the phone. If the phone was purchased with carrier financing, the carrier might refuse to unlock it until the contract is fully paid off – if even then.

How to unlock your iPhone for use with a different carrier - Apple Support

The person who sold you the phone should also carry out the steps in the following support article (if they have not done so already). This will not remove the carrier lock, but may prevent other problems (e.g., those from the phone still containing their data and being associated with their Apple ID).

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

The person who sold you the phone should have had the phone unlocked from the carrier that they were using.

The phone is going to be useless until you can contact the seller and have them unlock the phone for you.

We see stories every day from users who have bought locked phones and cannot use them.Might be best to simply return the phone for a refund.