Seems like they are right, LG's officially UNLOCKED phone for the US was the LG-US996 model. Newegg sold you an AT&T phone unofficially unlocked by a 3rd party (possibly refurbished). Once someone else touches it, LG can't guarantee it. You'll have to go back to NewEgg... LG claims that it was activated in 2016, they probably are right. Look at the listing on NewEgg, the phone was NOT sold by NewEgg but a 3rd party:
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The 3rd party seller can claim that it's new according to them, but in reality, nobody knows where it came from. Just like buying a used car, it has an unknown history from a possibly dishonest seller. Be careful, lots of fake Chinese clone phones out there... and 3rd party selling "new" phones that aren't actually new.