How can I unlock my iPhone 15 Pro Max after reported as lost?

What is the exact message that you see on the iPhone's screen?

Are you able to enter your Passcode? If yes, that will remove the phone from Lost Mode.

Otherwise, log into iCloud.com/find using your Apple ID and password, select the device, and choose "Stop Lost Mode"

That suggests to me that your carrier has blacklisted the phone.

If you tried to take the phone out of Lost mode – but the phone had no Wi-Fi or cellular Internet connection – the message would not get through to the phone. If your phone is blacklisted, that would be one possible, and likely, explanation for no cellular Internet connectivity.

As you were told earlier, if your carrier blacklisted the phone, only your carrier can take it off the blacklist.

Once they do, you can try logging into iCloud on the Web to take it out of Lost mode, as well.

Reported it lost to who, exactly?

If you reported it lost to your carrier, they have probably blacklisted it. If that's the case, only they can remove it from the blacklist.

When you first discovered that the iPhone was lost.......did you report this to your carrier......as KiltedTim has asked?

What happens if you log into iCloud.com/find using your Apple ID and password? Can you select the device, and choose "Stop Lost Mode"?

Your carrier does not need to "find" the phone to blacklist the phone from their office so the phone will not connect to their cellular service.

You need to contact your carrier again and ask them to remove the blacklist block on the phone. You will need to use another phone to do this.

Otherwise, you will not be able to remove the phone from Lost Mode.

You bought the phone from Apple.

What is the name of the carrier that you pay each month for cellular service? That is the company that you need to speak with.

The person who found the phone probably tried to reset it. not knowing that it would completely block it. and now everything I try to do tells me that it cannot find the device.

The person who found the phone would not have been able to try anything.....unless.....they knew your Passcode.

You were not still paying your carrier for cell service during the time that the phone was Lost, were you?

What did your carrier say when you asked them about "blacklisting" the phone?

FYI, this was how someone changed my password was they helped a friend with my email on find my iPhone app and changed all of my credentials thank God there was a fixed IP address they thought was hidden but there’s always a glitch somewhere