Getting 2 horizontal fields to stack vertically on mobile devices

Anyone out there have insight on how to build an email contact form using Contact 7 Form with Contact 7 Form Designer such that when you configure two fields per row for desktop version, those two horizontal fields will then display stacked vertically (in order) on mobile devices? I’ve built both the traditional version using DIV tags and another version done as a table. Neither seems to want to display all the contact fields in stacked vertical order (with reasonable margins on each side) using mobile devices. Any code information welcome.

Thanks!