Cesium for Unity can connect to anything that can serve 3D Tiles or quantized-mesh terrain.
With Cesium ion, you can download the 3D Tiles tileset it generates and use it offline via localhost or file:/// URL. You can also purchase command-line tilers from Cesium that allow you to convert data to these formats for offline and disconnected use.
We also license Cesium World Terrain and Sentinel-2 imagery for offline use. The Bing Maps imagery, however, is measured in petabytes, so using that offline is not realistic.
3D Tiles is an OGC community standard, so you can also find tools other than our own that produce content in that format, with varying levels of quality. I can’t advise on that because I don’t have first-handed experience with them and because I’d prefer anyway that you support Cesium for Unreal development by using Cesium’s tools. 
But we’re happy to see a robust ecosystem around 3D Tiles and don’t mind if others share their experience with third party tools.