How do I unlock an iPhone I bought from someone but can’t get hold of anymore

Sorry, you don't.

Activation lock is a security protocol that can't be bypassed. Only the person it is locked to can unlock it.

If you bought this from someone and it came locked, try to get your money back, you will never be able to use the phone.

The All Too Common SAD Reality of Buying a used iPhone - Apple Community

Nothing is going to happen until you are able to obtain the Apple ID and Apple ID Password of the person who originally set up the phone.

It is unlikely that the person that you bought the phone from has this information......because it was not likely their phone.

More likely, the phone is stolen. That is why you suddenly cannot get a hold of the person who sold you the phone. They have your money....that is all that they care about.

Try to get your money back. The phone is useless. Otherwise, turn the phone into the police or recycle the phone.

There is nothing you will be able to do to be able to use the iPhone without the original owners credentials. The link you viewed on the top of this page would only work if you had the original receipt from Apple showing your name and serial number of the device. Since this is a third party sale, you will not be able to use that link to remove the Activation Lock.