I need serious help! Prepaid go phone on iPhone 3GS?

I activated my att go phone prepaid sim and all that it works but when I put the sim in my iPhone 3GS it says searching… And then no service. I have tried everything
Turning off and on airplane mode
Disable 3G
Restore iphone
Etc.
I have put my brothers AT&T SIM card in and it works perfect. If I downgrade the baseband will it work?
Base band 5.16.08

I think you should get a new SIM card

The only way it could work if the iPhone is unlocked. You can go AT&T and ask them to do it. After they do it will work with any sim card anywhere in the world.

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