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Oh ok...well I don't know anything about playing it on a phone. In fact you are the first person I meet that is playing ESO on his phone
Did you try pressing all the other buttons. On PC it's the share button you have to press for getting on your mount. Other thing I would try, is looking for an app to calibrate the controller. Maybe you can switch buttons this way.
Yeah I'm going to try some other stuff to figure it out. I only really use it on the phone for writs and other stuff that don't matter to much. I haven't tried dungeons or pvp lol.