If the application requires ports to be opened on your personal firewall, then those same ports need to be opened on your router's firewall. I have to say though, such a requirement would normally be rather odd for a VOIP application.
To verify, keep your PC’s personal firewall up and BRIEFLY place your PC's IP address in the router's DMZ and see if it now works. If it does, then it’s a firewall issue w/ the router. If not, it’s something else. Remove your router's IP from the DMZ (since this isn't a good thing for the long term).
If it was an issue w/ your router's firewall, you now need to determine the port(s) it's using and port forward them from the router to your PC's IP address.
All this problems initially started after I uninstalled the software through Revo Uninstaller via advanced mode......
I use softphones too suggested by my VoIP provider (Onesuite) too from time to time for making my VoIP calls. And based on my experience, an error with Unable to Register to SIP server might or not might not be a firewall issue. Sometimes username or password is incorrect or you just can not connect to server (whether its on your end or your providers end)
Try it with another computer and preferably on another internet connection and see how it goes.