Never tried the mobile option. I liked how they used to handle it at MIA when the lines got long. If you had your physical AC membership card, there was a person with a clipboard who would write down your AAdvantage number and then send you in. The whole thing took about 20 seconds. It was much more efficient than the new process of scanning your boarding pass, staring at the screen and looking confused for a while, clicking a few times and then eventually admitting you.