I think I would check that the physical connection from the 9400 to the home network is reachable by the home network's wireless access point (so there's a path: 9600 -> wireless access -> wired access -> 9400). It's a long shot, but if your customer has done something funny with subnets on their switch/router, it's possible everything's connected but still won't communicate.

I would also connect a PC to the home network and do a "ping" to the 9600 and 9400 from the PC. If you get a ping reply from one, you would most likely get it from the other (unless there's some very funky networking going on that you probably wouldn't find in anyone's home). If you don't get ping replies from both, then there's a chance the customer's home network is your problem. If you do get ping replies from both then I would suspect a pronto-side configuration problem. I have multple 9600's and 9400's on my home network (all in network mode) and I can ping all of them from my PC.

You could also take a look at the diagnostics screen and see if that gives you any more info (Backlight+Menu+FirmKey2)

I don't know if this will help, but it might give you a couple more ideas.