Won't unlock a refurbished AT&T iPhone 6

Two years ago, I bought a refurbished iPhone 6s from Walmart. And I am using a Tracfone number with it. It works well. However, when I went to China and Canada, I found that I couldn't use sim cards of local carriers with it. It has sim card lock actually. Apple told me it's an AT&T device. So I tried to unlock it online; but it says "Looks like the device you want to unlock is active on another AT&T account...." I called AT&T representative. The representative said she had nothing to do with this. Several weeks ago I contacted AT&T via this twitter message. A social media specialist replied me that I had to contact Tracfone. I did so but Tracfone said this iPhone is originally an AT&T device, so the ball was kicked back to AT&T. Earlier today, I contacted AT&T via twitter agian. The reply is "In order for you to unlock the AT&T device, its a requirement that you have 6 months of service with AT&T first before you can unlock a device. Since you have never been a customer with us, you would need to talk to walmart to what they can do." I don't think it is fair that I cannot own the full function of the phone which I paid in full. I am not an AT&T wireless customer, but I am an AT&T internet and TV customer. Why do I, a helpless individual customer, deserve this? Is it because of technical matters or because of policy problems so that I cannot have it unlocked? Please let me know. And I would be grateful that you can help me out of this issue. Because I am going to Canada again in next week. I need my phone. Thank you.