I have much the same setup that you're looking for, just a stitch more advanced. I have webservers and such hooked up through a business class connection with static IPs and all that good stuff. But - this can still all be done through dynamic IPs as they really don't change all that much.
You already have a 'public' IP, you can ping it from somewhere else if you want. Almost all of the stuff that it does in 'stock' configuration has to do with you sending OUT requests, and it returns the data coming back IN. You need to tell it what to do with INCOMING requests for data. You can do that through port-forwarding, and DMZ. Now, be aware that some ISPs will block INCOMING requests to your IP address on specific ports (ie: 80 (http), 23 (FTP), 22 (SSH) etc), so you may have to configure your NAS to respond to different non-standard ports (ie: configure 8080 to http etc) but only do that as a last resort. Best to tinker with as little as possible. So here we go:
1) Quick n dirty: You're going to set your NAS as the DMZ computer. This means the router will forward all INCOMING requests to that internal IP address. You should configure your NAS to utilize a static internal IP address, this way if anything reboots, it will not shuffle your devices around. This means if your public facing IP address is 60.44.236.100 and your internal network is configured to 192.168.254.x (and your NAS is 192.168.254.100 for example) - --ANY-- INCOMING requests to 60.44.236.100 will be picked up by the router, and it will pass those requests to the DMZ computer 192.168.254.100 (your NAS). The good thing? It's easy. The bad? It exposes your NAS to anyone looking to try and hack your NAS. If you feel secure in how locked down it is, then that's the simplest solution. You can just log in as you normally would by pointing your browser at 60.44.236.100 (assuming a web interface and default http port) and it should come right up. It's really not safe unless you really know your stuff though.
2) port forwarding. You should also be able to enable 'port forwarding' on your router. This allows you to assign specific SERVICE PORTS for incoming requests and denies all other incoming. It also allows you to assign specific incoming service ports for different machines, and through some obfuscation provide a bit of security. Think of this as a phone-switch of sorts. Using the IPs from the previous example, you could enable port-forwarding where you designate an incoming port (lets use a slight obfuscation example) like 8080. You can then give it not only a destination IP address (your NAS for example at 192.168.254.100) but also translate the port to the standard HTTP one - in this case - 80. So - any incoming requests to 60.44.236.100 on port 8080 will be received, then aimed at the NAS at 192.168.254.100 and port translated to the standard http port of 80. Your NAS should then respond back, the router will translate it the other direction, and you should receive the response. The good? A little more secure than a full DMZ as it firewalls certain things unless you specifically enable a port translation. The bad? A little more work to set up.
There ya go. Your specifics will be very dependent on the cablemodem/router you have, so you'll have to get into that to configure it and that's way beyond what I'm getting into here. Hope that helps point you in the right direction though.
Just FYI, I actually have my own personal website, and I have it served in my basement (I have a small rack down there lol). I have a webserver (which also provides DNS), and a media server (running CentOS) which I've mapped INTO the webserver so I can pull up any of my videos from anywhere. I was stuck in the hospital last year for a week (complications due to chemo), and I actually connected my iPhone to the hospitals public wifi, connected to my website and was kept from dying of boredom by streaming Game of Thrones. 😀
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