.dropdown-submenu {
    position: relative;
}
.dropdown-submenu .dropdown-menu {
    top: 0;
    left: 100%;
    margin-top: -1px;
}
@media (max-width: 991px) {
    .dropdown-menu {
        border: none;
        padding-left: 20px;
    }
    .dropdown-submenu .dropdown-menu {
        left: 0;
        margin-top: 0;
    }
}