I have a G930A unlocked phone from AT&T connected on MetroPCS and I've checked in my settings for the update to Oreo and it says my phone is up to date and I even checked Smart Switch and still nothing.
Welcome to Android Central! Carrier-specific phones typically have to be on that carrier to get the official OTA update. So you'd have to get an AT&T SIM -- you could consider getting a pre-paid plan for just one month.