Hi,
In the settings of your module (in the joomla modules manager) you have a tab "HikaShop options" with an option called "menu".
This is where you can enter the menu item which will be used to display the products of the category the user will click on.
So if you don't see any product when you click on a category, it indicates that there is probably a problem with the settings of the menu item selected in that "menu" setting.
So that would be what to check.
If you want the module to use the same URL, what you need to do is to enter that main URL in the "canonical URL" setting of the category (it's also important for SEO). And then, you can activate the "force canonical URLs on listings" setting so that the system will use the canonical URL of the category instead of trying to generate dynamically a URL based on the "menu" setting of the module.