Unlock G900A
 

What is the method to unlock AT&T G900a? I need to be root? because this model does not have root

I don't believe so, just read helpful notes

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@eduard504
Hello
Please read how to use newqc unlock helpfile for all samsung qualcomm sim unlock and msl reset.

if its show unsupported baseband try downgrade baseband only option for you.

Bro, if you need root you can downgrade to NB1. This is not a problem.
Find the NB1 odin tar. You need Odin 3.0.4 to downgrade. Root the phone, unlock and then if you want reflash the phone to remove the root and take the ota again to 5.0.1 OK2 and done.

Edited again
"Sorry my mistake, i thinking in i337, this steps can't be done to a G900A"
Please someone can delete my message

do the steps, but never activated the DIAG port