Hello,
Currently WMTS is not supported with Cesium for Unreal. However, it should be possible to create a WMTS Raster Overlay based off of the existing overlay options, if you’re experienced with C++. You can find more information here: Adding WMTS served by geoserver into UE - #4 by Kevin_Ring
If you do create this functionality, please consider contributing it back to the plugin by creating a pull request.
-Alex