You can NOT use set master behind a router, as the router is your point of exit from your home network, NOT THE MACHINE THE SERVER IS RUNNING ON!
If you have correctly opened up the port 27015 (Thats the only one you need to open if your using the default server start up port) then your router should be forwardiing all traffic to the machine you have the server running on. Make sure the port 27015 is open on the router, and that port forwarding is set up to forward the traffic to the machine your server is running on!
So for example if your router gave you the IP 192.168.0.1 for the machine thats running the server, and your external IP (For your router! is for example 120.34.66.7
then your friends will need your external router IP to connect to your server, and not the IP that the server is actually running on.
If the router if set up correctly for port forwarding, you should now be listed on the internet aswell as your friends being able to connect.
If you have real problems with setting up port forwarding, then set up the router to make the machine with the server running a DMZ. This has some security risks, and I would advise against doing this, but if all else fails, at least try it.
Ancient