@media screen and (min-width: 991px) {
    .ylex-ad-mobile {
        display:none;
    }
    
    .ylex-ad-desktop {
        display:block;
    }
}

@media screen and (max-width: 991px) {
    .ylex-ad-mobile {
        display:block;
    }
    
    .ylex-ad-desktop {
        display:none;
    }
}