I would suggest that it is down to your phone design and you needing to select the user input text box itself so that the phone knows to bring up the keyboard for you to type with.
https://www.wix.com/corvid/reference/$w.FocusMixin.html
Even though the focus function should…
Receiving focus through a call to this function is equivalent to a user clicking on or tabbing to the element manually.
It might be that this function does not work on Iphone mobile browsers like Chrome or Safari and you have to manually press the text box to call up the keyboard.