A bunch of users over at the forums are confirming that the Android 5.1.1 OTA update has begun rolling out for their LG G Flex 2, model no. H950. AT&T and LG are silent thus far on it, but AT&T to come out with an official word pretty soon, with a full changelog.
For now, you can force your AT&T G Flex 2 for the update. For this, open Settings > About device > Software update and then tap on ‘check for update’ button. Hopefully this also brings the Stagefright fix that many Samsung and Nexus devices have already got.