Configure Synology NAS, Netgear modem/router for remote internet access to files

Hi, I have a NAS drive (Synology 2-bay DS212) that hosts my files at home and is accessed on my local network from desktops, laptops etc via mapped drives. I want to be able to see these files (preferably via a mapped network drive) from offsite computers that are connected to the internet at large (but are not on my local intranet). All computers are Windows 7. I have a cable modem (Netgear CG3100) which is also a 4-port and wireless router. The NAS drive is plugged directly into one of the 4 ethernet ports. I have an 8-port TP Link switch which has some other gear on it (voip phone etc), but the wired desktop connections do not use this - they go directly to the 4-port modem/router the same as the NAS. I have a semi-static IP address. That is, it doesn't change often, but it CAN change. I have a couple of different dynamic IP forwarding setups (DynDNS, [login to view URL] and [login to view URL]). The NO-IP seems to require a downloaded service to run permanently on a PC connected to the network so that it can check for changes to the IP address. I would prefer not to have this as I would like to be able to turn my PC's off and still be able to access files on the NAS remotely. I am not sure how DynDNS and [login to view URL] keep track of changes to the IP address, but if they do it differently, that would be preferable. I have tried all sorts of different "automatic" and manual router configurations using the EzInstall option or trying to create Port Forwarding rules on the router, but can't seem to get things to work. The best I can do is to get remote access to port 5000 which allows me to log in to the Disk Station Manager, but I can't seem to get any of the other ports to "open". I think 5005 is the one that I really want which provides what Synology calls WebDAV access - and the description leads me to believe that this is what I want. I have also tried installing Synology's EzCloud package and the required installation software on a remote computer, but could not seem to get files at either end to see, let alone sync with each other. Not sure if this was something to do with the lack of properly open ports or whether I just stuffed up the EzCloud installation entirely. I am now willing to pay some intelligent person to give me the solution I need - as I am wasting far too much time on this! Let me know if you need any other information at all. First working solution (with a reasonable price) gets the job. Thanks for your help!