Yes the One UI Update is out for all Note 8. For At&t devices in order to get OTA updates you'll need At&t service or At&t Sim in the device if not the update won't show up. If you don't have At&t service try to use Samsung Smart Switch on PC and see if the update pops up. If not your best option is to get hold of At&t sim and insert in the device and update shows up or Maybe go to authorized Samsung shop or Best Buy they can update the device.
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