Verification Loop - when setting up a new iPhone

I recently switched carriers from Straight Talk to AT&T, but I was always using an AT&T iPhone 7. When I switched over I kept my same phone number which AT&T ported onto my new sim card for me. When I inserted the new sim into my phone, it asked me to set it up like new. After I finished entering my Apple ID & password & such it goes to my home screen for a split second & then loops right back to the setup screen. I have tried everything, I shut it off & turned it back on, I reset it by holding the volume down button & the lock button, I reset all settings & set the iPhone up as new, I hooked it up to iTunes & reset it that way & absolutely nothing worked. But as soon as I remove the sim, it goes to my home screen as if no problem is there. I have put other peoples sim cards in the 7 & the same issue happens every time. In my desperation I went to Simply Mac, they sent my phone off & I received a brand new one....got home to set it up & the exact same problem occurred. THE KICKER ---- it is not my sim card! I put my sim into an iPhone 6 I an iPhone 5s & have service & a functioning phone. All of the technicians at Simply Mac have never seen this issue before & I am stuck. My phone was only 6 months old & in perfect condition....any advice please....

Johnnie xoxo