Check out this article from ATT about your phone and updates: https://www.att.com/device-support/article/wireless/KM1253489/Samsung/SamsungG965U
Since firmware updates are carrier specific, you might be facing an issue with updates from them.
I'd encourage you in this case to reach out to ATTif you are using one of their devices. >>> ATT <<< support can be reach using that link.
@userZlDdV0Du7f let us know any updates here!