Sim unlocked or not

I keep reading everywhere that i can put my att sim in this thing and it will work. Unfortunately i dont have a active att simm. All the ones i put in say sim is from u known source. No biggy. Seen that before. But my prob is no signal. I goto net selection and i see verizon and att but it wont let me register on the network. Im assuming this is due to the inactive sim i have. No rush on this droid ultra is backup pho e if possible. I run a lg g4

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