I resolved my issue and sharing what I did in case it helps anyone else.
I'm new to Radzen and new to Blazor, so this might be really basic.
For some reason, I had to add a @using for Microsoft.AspNetCore.Components.Web, despite the fact that this already exists in my _Imports.razor file. After this, it works as expected.
And finally, I now realise that _Imports.razor applies to the folder it's in and all subfolders. My pages were in a different folder structure, so they weren't picking it up.