If the stationary is a value why don't you pass it as a parameter like this:
from a in Products
where (from b in Categories
where b.Category_Type == sampleParameter
You only need to put the user input into the sampleParameter parameter. The dynamic linq is used to dynamically switch between the things you query. So for example you can once query against the Category_Type and the next time query on Category_Id. As for your case if I have understood it correctly I believe you don't need to use any dynamic Linq.
the Telerik team