I thought I would post what worked for me although, I was trying to override a non-generic route in the public store. I am using nopCommerce 4.10
I was facing a similar issue with trying to override the "/customer/downloadableproducts". I tried various ways to override the route, I tried to register the custom controller, which inherited from the CustomerController. None of those methods were working.
I finally got it working by using the Route attribute:
on my custom controller action method. I didn't register a new route, I didn't register the custom controller and I didn't inherit from the original CustomerController. Hopefully this helps someone.