Why can't I update my S6 edge plus?!!!


    Hi everybody, I'm from Africa and I recently bought a used Tmobile Samsung Galaxy S6 edge plus (sm-g928T) but I can't do software update.

    When I go to Settings > About device > And click "Download updates manually"

    It says "Registering device..." and then it either says "Processing failed." or "Registration failed."

    can you help me to fix this problem please?

    I know the phone came from USA in an unofficial way but it's not lost or stolen, I checked it on Tmobile website using imei number and it said it's "clean" device.

    I did a factory reset and it didn't change anything.I tried smart switch and it showed message: "Your device software is up to date: PG1(TMB)" which doesn't make sense because my phone in on android 6.0.1 while it should be on android nougat

    Here's what I got in "about device"


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