I think I've solved it with this code:
/* hide testimonials mobile */
@media screen and (max-width:640px) {
div#block-yui_3_17_2_1_1633431612221_11185 {
display: none;
}
}
/* hide testimonials desktop */
@media screen and (min-width:641px) {
div#block-yui_3_17_2_1_1634009578077_3559 {
display: none;
}
}
Can you please double-check it works OK if you get the chance? 🙂