Unable to update phone software on Samsung Galaxy S5, dialogue wrongly says "software is up to date"
I'm having an issue updating my Samsung Galaxy S5 (SM-G900A), which is currently running Android 5.1.1. If I attempt to update, via Settings -> About device -> Software Updates -> Check for updates and confirm that I want to check the AT&T server for new software updates, I get the message "Current software is up to date."
This is, however, not true: I am running baseband version G900AUCU4CPA3, and consulting https://www.att.com/device-support/article/wireless/KM1135275/Samsung/GalaxyS5G900A/ shows that there have been nine updates since then, including an update to Marshmallow.
How can I update the software on my phone? Any help would be appreciated!
Some possibly relevant information: I purchased the phone from a third-party retailer in the US (Amazon), but I have never had an AT&T contract with the phone. Also, I am now living overseas. (In particular, I saw a post about a similar issue for another model of Samsung phone, and one recommendation there was to take the phone to Best Buy to have the phone manually flashed. As I won't return to the US until later this year, that is not a reasonable option in my situation.)