please add a second element to this script, name it fhFixedButtonMobile and give it the same values of fhFixedButton, make sure to append both elements
   

*******
<script> 
    var p = parent.window.document;
    fhStyleSheet = p.createElement('link');
    fhStyleSheet.href = 'https://fh-kit.com/buttons/v2/?red=cc0000';
    fhStyleSheet.rel = 'stylesheet';
    fhStyleSheet.type = 'text/css';
    p.head.appendChild(fhStyleSheet); 
    fhFixedButton = p.createElement('a');
    fhFixedButton.href = 'https://fareharbor.com/embeds/book/darkes/?full-items=yes';
    fhFixedButton.className = 'fh-button-true-flat-red fh-shape--square fh-fixed--side';
    fhFixedButton.innerHTML = 'Book Now';
    fhFixedButton.style = 'letter-spacing: 1.5px !important; padding: .3em 2em !important; box-shadow:none !important;';
    p.body.appendChild(fhFixedButton);
</script>
*********** 


<script>
    var p = parent.window.document;
    fhScript = p.createElement('script');
    fhScript.src = "https://fareharbor.com/embeds/api/v1/?autolightframe=yes";
    p.body.appendChild(fhScript);
    fhStyleSheet = p.createElement('link');
    fhStyleSheet.href = 'https://fh-kit.com/buttons/v2/?color=F113B5';
    fhStyleSheet.rel = 'stylesheet';
    fhStyleSheet.type = 'text/css';
    p.body.appendChild(fhStyleSheet);
    var fhFixedButton = p.createElement('a');
    fhFixedButton.href = 'https://fareharbor.com/embeds/book/levelupwarrington/?full-items=yes&flow=1049091';
    fhFixedButton.className = 'fh-button-true-flat-color fh-shape--square fh-icon--cal fh-fixed--bottom fh-hide--mobile';
    fhFixedButton.innerHTML = 'BOOK NOW';
    fhFixedButton.style = 'letter-spacing: 1.5px !important; padding: .3em 2em !important; border: 1px solid #FFFFFF !important; box-shadow:none !important; left: 20px !important; right: inherit !important;';
    fhFixedButton.id = 'fhFixedButton';
    var fhFixedButtonMobile = p.createElement('a');
    fhFixedButtonMobile.href = 'https://fareharbor.com/embeds/book/levelupwarrington/?full-items=yes&flow=1049091';
    fhFixedButtonMobile.className = 'fh-button-true-flat-color fh-shape--square fh-icon--cal fh-fixed--side fh-size--small fh-hide--desktop';
    fhFixedButtonMobile.innerHTML = 'BOOK NOW';
    fhFixedButtonMobile.style = 'letter-spacing: 1.5px !important; padding: .3em 2em !important; border: 1px solid #FFFFFF !important; box-shadow:none !important;';
    if (!p.getElementById('editor') && !p.getElementById('fhFixedButton')) {
        p.querySelector('.widget-html-html-1').style = 'height:0px !important;';
        p.body.appendChild(fhFixedButton);
        p.body.appendChild(fhFixedButtonMobile);
    }
</script>