If you're adding your own layer on top of CesiumJS's base layer, then it will still be contacting the Cesium ion server for the base layer. So make sure to either replace the base layer with the offline imagery (like in the offline example) or remove the base layer before adding your own.
See also the "common mistakes" section on this page, which explains that constructing the viewer with the default options will load the Cesium ion Bing imagery by default, so that will show you a request to the ion servers in the network tab, even if the layer in the next frame, but that shouldn't prevent the globe from rendering: