brand bew Iphone 12 pro max has physical sim from At&t

Its very possibly brand new yes. Just that it's not meant to be sold outside of the U.S. As such the warranty likely started when it was sold by Apple to whoever took it to Dubai and sold it to you.

The SIM card is placed in the iPhone by Apple during the manufacturing process and packaging. Not by a carrier.

Yes. U.S models will have a sim card for the selected carrier.

Apple does not actually sell U.S models abroad, so any phone you see that says U.S model will be one meant for the U.S so will likely contain a SIM card for a U.S carrier.

U.S models abroad are considered gray market devices, and Apple may not honor the warranty and if they do, it would have to be taken to the U.S for service.

You purchased a gray market phone. It was designed to be sold in the US as an AT&T device. My guess is that you purchased from someone that is not an Authorized Apple Retailer. While it may be unlocked, you will not be able to get it warranty serviced outside the US. You would have to bring it to the US for service.

ChrisJ4203 wrote:

You still must be ordering from a gray market. Apple does not ship US phones outside of the US, especially to an Authorized Reseller.

This is correct. And as such warranty will not be valid in Dubai, and will likely have already started to run.

It's never a good idea to buy such devices.

You bought it in Qatar?

I've seen 3 other posts just today about these devices and they all appear to have been stolen.

Take it back and demand a refund.

Why? Because you bought a gray market phone from the US market. It is not "new". It stopped being new when it was originally sold in the US.

Apple doesn't ship outside the US and its territories.

I would just consider it stolen.

You still must be ordering from a gray market. Apple does not ship US phones outside of the US, especially to an Authorized Reseller.