I use the Container Station wizard/UI as it provides all the required options and saves having to putty or SSH into the QNAP and run docker commands.
If you're able to connect internally it sounds like you're just about there, and the only thing left to do is sort networking.
I have my Container Network mode set to Bridge and use a static IP to set a unique address for the MC instance on my network. Once it is in this state, MC is behaving no differently to how it would on a physical host on your internal network. If you use DLNA quite a bit in your setup you might want to use DHCP instead, and reserve the address for MC to use. DLNA often works better that way but I find it makes no difference in my setup so leave it at static. Pretty simple, have attached a screenshot.
Only thing to remember is to also add a Mac Address as noted in this thread, I've scrubbed mine out in the screenshot.
Just remember as well to back your library up before making a change to a container setting, as it will most likely cause the image to generate a new container.
To access MC from outside your network you either need to configure port forwarding on your router, or possibly DDNS from within the QNAP.
I use port forwarding, and configure anything coming in on port 5800 to go to the IP Address I configured in my Container Station setup. Similarly for JRemote I configure port 52199.