Hello @sportyxl,
Thank you for using Simple Side Tab. Thank you for providing the link to the site. I was able to see what the issue is.
The tab is set to start at 600px from the top. This is too low for mobile views. You have two choices: lower the Top value from 600px to 350px in the settings, or apply CSS to only target the mobile view. If you choose the later, then you can use something like this.
@media screen and (max-width: 480px) {
#rum_sst_tab {
top: 350px;}
}