1. A concise explanation of the problem you're experiencing.
2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.
3. Context. Why do you need to do this? We might know a better way to accomplish your goal.
4. The Cesium version you're using, your operating system and browser.
Cesium recently added support for Pelias (https://pelias.io/) an open source geocoder which should let you do what you need. I think what you’re looking for is reverse geocoding (https://github.com/pelias/documentation/blob/master/reverse.md).
And then check the Cesium docs for how to use it with Cesium: https://cesiumjs.org/Cesium/Build/Documentation/PeliasGeocoderService.html
I hope this helps!