@tuanphan I am sure you're tired of answering this same question, but I am having the same issue with the banner on my homepage and not being able to change the focal point on the mobile version. I would like the banner image on the home page to still span the entire height of the device, but would like to shift the focal point to the far right so that the family fills the screen.
My website is www.nizhoniphotography.com and the password is Password123. The banner image I'm referring to is the one on the home page. (though I wouldn't turn down a better solution for the appearance of banners on the Details and Contact pages for mobile/tablets)