Can you do normal SSH into your mac and the tablet? Try that first. Also, your router might be in "stealth" mode. It sees if a local computer (iMac) is listening to a port, and only then opens it. I hate stealth mode. For example, my computer has "Intel Active Management", which is a thingy that runs when the computer is turned off or on. It can remotely monitor and switch on your computer. BUT, because it is very passive, the stupid router blocks the ports!!