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