Unlock Iphone

Hi!!

I had an unlocked iPhone 5 and joined AT&T Go Phone Plan starting from August, 2014. I bought a new pre-paid iphone SE on April 9, 2016. I'm still using the same phone number and same account. However, when I tried to unlock my new SE, it said my device is active on a different customer's account. I don't know what this means...I've never let anybody else use my account, phone or number. Is it something that's related to my old iPhone 5? I'm not even using my old iphone anymore so I'm not sure what it meants by saying "it's still active on a different account." And also, I went to the AT&T store twice this year to try to find out what's going on. They told me that my devide has not been active for at least 6 months of paid service. So I've been paying my bills for the past 6 months using the Go-Phone $2 daily plan. I called AT&T and they told me that the daily 2$ plan works out fine and all I gotta do was to wait for 6 months. But it still denied my request even though it's after 6 months' paid service already. Can anybody help me?

Thank you so much!