Is there an alternative to MapQuest tiles?

Yes, Mapbox should work, and should be the default in a soon to be released new version of DSS.

You can also register an API key on Mapquest and use this template:

in the html tab of the webapp:

<script src="https://www.mapquestapi.com/sdk/leaflet/v2.2/mq-map.js?key=YOUR_MAPQUEST_KEY"></script>

in the JS tab:

// instead of 
//var map = ...
// (...)
//map.addLayer(omq);
// use:

var map = L.map('map', {
  layers: MQ.mapLayer(),
  center: [ 42, 2 ],
  zoom: 13,
  opacity: 0.5
});