The new UrlTemplateImageryProvider handles subdomains, however it is not the case of
The WMS provider misses a ‘subdomains’ option in the constructor that could be passed to the UrlTemplateImageryProvider constructor.
For WMTS, subdomains handling can easily be added to https://github.com/AnalyticalGraphicsInc/cesium/blob/1.13/Source/Scene/WebMapTileServiceImageryProvider.js#L157.
Is it the right approach? If yes I can create a PR for this next week.