The key to doing this is going to be using AJAX of some flavor -- Microsoft.Ajax or jQuery Ajax. If the UpdatePanel is not working, then I'd suggest using jQuery to submit back to the server using AJAX. This would involve creating a WebMethod to accept the AJAX post on the server side and instrumenting the client-side with jQuery to send the request/receive the response. Without seeing your HTML it's a little hard to be very specific.
Basic idea:
$(function() {
$('#modalSubmitButton').click( function() {
$.ajax({
url: 'path-to-your-web-method',
dataType: 'json', // or html, xml, ...
data: function() {
var values = {};
values['field1'] = $('#field1ID').val();
...
values['field4'] = $('#field4ID').val();
return values;
},
success: function(data,status) {
... update page based on returned information...
}
... error handling, etc. ...
});
return false; // stop any default action from the button clicked
});
});