I had this issue on a phone I was working on yesterday. I performed various tests including a new charge port with a new antenna with no success. It was a locked device so I couldn’t test it with a new sim card. It just suddenly began working again after trying for 45 minutes. Our master technician said it could have been a bad IC chip, which reballing wouldn’t have solved and it can’t be replaced. He also said that iPhone 6s, 6s plus, 7, and 7 plus have been known to have this issue every once in a while.