From your css file, a few changes that is needed, I did not test but this should work -
.dropdown:hover > .dropdown-menu,
.dropend:hover > .dropdown-menu {
position: block;
margin-top: .125em;
margin-left: .125em;
}
@media screen and (max-width: 768px) {
.dropend:hover > .dropdown-menu {
position: absolute;
top: 100%;
left: 0;
width: 100%;
}
}
I have changed max-width instead of min-width, the position property is set to absolute to position the submenu, top is set to 100% to place it below the parent menu, left is set to 0 to align it to the left side of the parent menu, and width is set to 100% to make the submenu take the full width of the screen.