Hello,
I have followed this help article, how to bind CheckBoxGroup to model on razor page. This article does not show how to get selected values back on post.
https://docs.telerik.com/aspnet-core/html-helpers/editors/checkboxgroup/razor-page
OnPost CheckBoxGroupModel models that is bind property, it's CheckBoxGroupValue property stays null when posted back. Only way to get values back is to read them from Request.Forms["checkboxgroup"].