I looked through the resium website and was not able to find a community forum. For bugs and issues, my best guess would be to post them on their GitHub page. With regards to questions, I would recommend first going through the getting started guide. If your question persists, you should post it as a GitHub issue.
I am not a member of the resium development team, these are just my suggestions as a Cesium dev and someone who has read through their website. When in doubt, resium’s contribution page is a great resource for these types of questions. Resium-related posts on the Cesium Community Forum are also a terrific resource. Here are a few of my favorite threads:
@Mark_Erikson seems to be a particularly knowledgeable community member on this topic.
-Sam