mobile

I have had my HTC One M7 for just about 2 years now, and my contract with AT&T has ended. Since my contract has ended, I went on the AT&T website and sent in an unlock request for my phone. I entered in all my info, including the IMEI number. The next day, I received an email from AT&T saying that they couldn't unlock my phone because the IMEI # wasn't recognized as an AT&T device. Now, I'm not sure specifically how my IMEI was changed but it was probably from flashing a bunch of custom roms and kernels. I still have the original IMEI # from AT&T on the sticker on the back of my phone. What can I do now the fix my IMEI back to what it originally was so I can get my phone unlocked? Any help would be appreciated.