So I bought a AT&T Unlocked Note 8 and imported it in Europe.It's been months and I still haven't gotten the Oreo update even though it should've came by now so I'm wondering if the issue is using a US phone in Europe.
US carrier branded phones only receive their updates if there is a carrier sim in the phone. If your phone is from AT&T, it will only get an update if there is an AT&T sim card in it. Unlocking does not remove this restriction on them.
I am not even sure that phone can be rooted. However you don't need to root to install updates manually. You can try if SmartSwitch for PC can get around this restriction as it will download the update file to the PC and update the phone using that program. Or you can get the file from Sammobile and use Odin.