will att software updater only work with att sim?

My nexus 5 recently crapped out on me and I picked up an at&t s6 edge that I unlocked to use on tmobile.

Obviously, its night and day compared to my nexus as far as development..

Anyways, when I go to at&t software update on the phone it says I am up to date, but im on the old OE2 build and want to update.

Will it only update if Im using at&t sim? or is something else wrong?