I can't play "online" but I can connect using the connect command, might work for you too? The whole command is "connect xx.xx.xxx.xx:28960 where the x's are the IP you're connecting to.
I can't host with my ISP unless I pay extra for a static IP, but the person I regularly play with can host no problem. If you have everything in the "Network" section of the F10 menu looking good you're probably fine to host and have people connect, mine shows UPnP disabled even though it isn't. You'll type in the public IP and not the local one for the command