I bought an iPhone and it has remote management

You can only disable it if you have an original receipt from Apple. Otherwise, you will need to contact the organization that manages the device if you purchased it from them. If you purchased it from someone that is not Apple, and is not the organization that manages the device, you will need to contact the seller for a refund.

That does not mean it isn't stolen. That just means it wasn't reported as stolen.

It is managed under Apple's business manager program (formerly the Device Enrollment Program).

Either it's stolen, or it was improperly disposed of by the company that originally purchased it.

Either way, it's not usable as it is and Apple will not intervene.

Next time, buy from an authorized retailer.

You may be able to contact the organization that manages it and ask them. Be aware that they may demand the device to be returned to them with no compensation.

Turn it over to the police. You purchased a stolen phone.

If you don't return it, they can block it as well as pursue legal options against you. If they disposed of it improperly, they can easily release it to you for use remotely.

They could track the location of the device if it was stolen. If you try to set it up, it will also alert them.