If you have a router that does NAT (Network Address Translation) and your UltraVNC viewer and server are on different sides of the router,
you won't be able to establish an UltraVNC connection without some additional configuration.
There are several options depending on what's possible in your scenario.
In case you would like to remote control without any software installed on the target computer you need UltraVNC SC. SC is for SingleClick, i.e. the user on the to be controlled computer needs to simply click on a web page and remote controlling begins.
With the help of the repeater you can use UltraVNC viewer behind a NAT router.
The NAT to NAT connectorfor connections between UltraVNC viewer and server behind NAT routers without any router modification.
If you have access to the NAT router, you can configure it to forward UltraVNC ports. That way you don't need a repeater.
UltraVNC SC (SingleClick) is a customizable mini UltraVNC server for download. This is very convenient for help desk support because the customer doesn't need any preinstalled remote control software. Just have him or her download UltraVNC SC and start remote controlling. At the end of the remote control session UltraVNC SC deinstalls itself and leaves the computer without additional software.