The iPhone 11 has a viewport width of 414px so try @media (max-width: 415px)

I also think your selectors are mismatched:

try this simplified selector :

@media (max-width: 415px) {
  #slider-9-slide-11-layer-1.rs-layer {
    font-size: 20px !important;
    margin-top: 100px !important;
  }
}

Also check if you are using the correct css file