Thanks for writing in! Regretfully the category filter is only available in the blog index when you enable the Filterable Index option. For more details about this option, you can check it here: https://theme.co/apex/forum/t/customizer-blog-options/137
At the moment, we do not have any feature that offers you to add a tag filter in a category archive. What you can do is to create a page and use a grid to display the post items in one category. You may use Essential Grid or The Grid plugin which you can install in X > Validation > Extensions or Pro > Validation > Extensions. When you create a grid, there is a grid option that enables you to include a sort filter within your grid. For more details about this grid plugins, please check it here: https://theme.co/apex/forum/t/extension-essential-grid/68, https://theme.co/apex/forum/t/extension-the-grid/74
Hope this helps.