Haven't Had A Security Update Since 10/1/2017

I have a Samsung Galaxy S8+ that is AT&T branded and I have AT&T service on it. I'm still stuck on Android 7.0 and the latest security patch I have is from October 1, 2017. There is an exclamation mark next to the update option at all times but always tells me my phone is fully up to date. My baseband version is G955USQU1AQJC. I bought this phone back in 2017 thinking I'd be able to update my phone to a new version of Android but it just seems AT&T may be blocking me from that.