As i see it this is not a Dokuwiki nor Synology question, but has only to do with the router.
Did you read/do this ?
http://www.synology.com/en-us/support/tutorials/456
As a temporary workaround you could put the Synology's IP adres in the DMZ and see what happens, the webserver should respond when you adress the external WAN adress of your router, also
http://yourWAN.ip/ or even better http://yourWAN.ip/dokuwiki/doku.php
http://en.wikipedia.org/wiki/DMZ_(computing)
Your webserver listens to http-requests on port 80.
If this works you should create a safer setup.