I am having trouble with ARD 3.8 linking via a Cisco modem/router

Both I and my parents iMacs are as updated as can be with ARD ( Admin and Client), Yosemite,etc. I have configured their Cisco modem/router to allow the appropriate ports to be forwarded. When I look to see if their computer is available it shows "Offline". Even though they are online and I can actually link to their computer. What should I be looking for in the configuration to allow me to see they are actually available?