Unlocked AT&T LG G3 Cannot Receive Lollipop or Marshmallow Update
I recently purchased an unlocked AT&T LG G3 (D850) which has Android 4.4.2 installed. I have an AT&T prepaid sim card inserted in the device but I am unable to receive any software updates. I tried many times to check updates through Settings -> About phone -> Software Update -> Update Now but it always shows that the software is up-to-date. I also reset my phone multiple times and even brought the device to AT&T Device Support Center but they said they do not have the tool to flash the update.
Any solution or ideas?