I understand your concern with this and I would like to confirm that manual time is not protected under our Upwork Hourly Protection.
You can also consider to switch to a Fixed price contract when your work is done on your mobile device. To learn more about how you're protected under our fixed price protection for freelancers, check out this Help Article. Thank you.
I am really sorry to say that I also don't find any solution to this problem. You can use your computer for your job. So that you can track time with the desktop app. Or you can charge project basis. Best of luck to you.