iPhone 4 for at&t iOS 5.1 bb 04.12.01

hi have iphone 4 16 gb at&t
want use tmobile
many told me it's impossible jailbreak , unlock
jailbroke , have cydia installed on it.
(i think it's tethered ? every time reboot have plug computer , go dfu mode redsn0w in order use cydia)
anyway i'm trying unlock , i've tried everything.
have tried downgrading bb , ios it's not working
don't have shsh blobs , don't know how them.
please me!!!

 

there no unlock possible.

 

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