Sorry for the delay - this is not a forum I frequent... Whenever you go to create/edit a Docker container, there are a few default settings that are hidden away inside the show Advanced Options menu. Specifically, the WebUI: option will be prefilled with [hostname]:[port]. Delete that and actually type out the IP address of your NAS and the container's port (ignore the fact that Calibre has multiple WebUI ports in my screenshot). This is likely just some buggy code in the plugin that's not correctly reading the default values.