I can not connect to my mac g5

I can not connect to my mac g5

Hi all
I have a problem since I bought the option for and since I was free. I can not connect to one of the three computers I have. All three are Mac and two connect well but one I can not. The computer does not connect has not generated a VNC connection. I have created a manual connection and I can VNC to connect to it if I'm in wifi near him but from my house I can not. Does anyone know how I can fix it? The funny thing is that when I'm near the computer I can connect with but if I'm home is when I need I can not.