First check the IP of your Synology NAS, and your public IP (the one you are connected to the internet with, http://www.ip-adress.com/
could help you to find it). Note : make sure that Synology's device IP will not change ! For this, either use static IP in Synology's configuration or enable DCHP reservation by MAC address in your router.
For exemple purposes, we will use 192.168.1.23 for Synology's device IP and 22.214.171.124 as a public IP !
Now open your router configuration page (check your router user guide for how to do it).
In your router configuration page, configure NAT forwarding (also called port forwarding). In this exemple I'll configure the router so the Synology device will be available when connection to 126.96.36.199 at port 1234 :
As local IP address, set 192.168.1.23 and as local port set 5000 (or 5001 if you plan to use SSL), as public port set 1234. If you have specific configuration for both TCP and UDP protocols, fill both protocols with the same informations !
You may have to reboot your router for the new configuration to be applied !
Now use your web browser and check that you can connect to http://188.8.131.52:1234
(your public IP and public port), use https if you are using SSL. If everything if ok, then in SynoDS enter the following informations :
Address : 184.108.40.206 (your public IP)
Port : 1234 (your public port)
and SSL on or off depending on what you have configured previously