My 6s Plus has been going down to 4G (with AT&T) all day. I've tried rebooting, and resetting network settings. It stays on LTE for a few minutes, then goes down to 4G. However, other iPhones (5S and 5C) in the office are still on LTE. Not sure if it's AT&T (maybe a new carrier update is needed for the 6s Plus?) or an issue with my iPhone. Anyone experienced this?