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https://www.elegantthemes.com/blog/divi-resources/how-to-change-divis-column-stacking-order-on-mobile-devices

@media all and (max-width: 980px) {
/*** wrap row in a flex box ***/
.custom_row {
  display: -webkit-box;
  display: -moz-box;
  display: -ms-flexbox;
  display: -webkit-flex;
  display: flex;
-webkit-flex-wrap: wrap; /* Safari 6.1+ */
flex-wrap: wrap;
}
 
/*** custom classes that will designate the order of columns in the flex box row ***/
.first-on-mobile {
-webkit-order: 1;
order: 1;
}
 
.second-on-mobile {
-webkit-order: 2;
order: 2;
}
 
.third-on-mobile {
-webkit-order: 3;
order: 3;
}
 
.fourth-on-mobile {
-webkit-order: 4;
order: 4;
}
/*** add margin to last column ***/
.custom_row:last-child .et_pb_column:last-child {
margin-bottom: 30px;
}
 
}