The "Service" is just there for convenience. For example, if you chose Personal Web Sharing, it forwards port 80 TCP to port 80 TCP. You don't need to choose a service. We won't in these instructions.
But let's first go back to the Internet -> DHCP setting. If you haven't already made it fixed to an IP outside the pool, let's establish a DHCP Reservation for your DiskStation within the range of the pool, . Note the Extreme itself uses 192.168.1.1 so the range starts at .2 and runs through 200. If you configured the DiskStation for a fixed IP within the pool, just reserve it here. Same if you left the DiskStation set for DHCP--choose an IP within the pool for it here. You can base the IP on MAC address. Hint: don't choose an IP that's already in use by a Mac or PC.
Now that we know for sure the IP of the DiskStation, we can go back to Advanced -> Port Mapping, and do port forwarding. Click + to create one for each port you need forwarded to the DSM. Here are the ports with suggested "Description." Don't forget to correct the default IP address in each one to the one for the DiskStation. Here are some basics. The rest are here
. Use TCP and leave UDP blank.
5000 DSM (management console and Audio Station)
5001 DSM HTTPS (management console and Audio Station)
5005 WebDAV (to support the DS file app or CalDAV)
5006 WebDAV HTTPS (to support the DS file app or CalDAV)
5007 Alternate (if you set this up for Web Service and PhotoStation)
7000 Files HTTP (for File Station)
7001 Files HTTPS (for File Station)
22 SSH (hint forward 22022 to port 22 and avoid having brute force attacks that eat up bandwidth)
80 HTTP (for Web Station and PhotoStation)
443 HTTPS (for Web Station and PhotoStation)
For security when accessing your DiskStation over unsecured wireless networks, you may choose to only open the HTTPS version of these ports, or you may, where possible, leave both open and redirect the HTTP to HTTPS in the DiskStation. You may also choose to forward none of these and set up a VPN on the appropriate port. You can always go back and disable any port forward you set up by unchecking it.
This is a wonderful router and fairly easy to use. Don't be surprised when it takes up to a minute or more to "Update" the settings. Let me know if you need more help.