Open Macintosh HD > Applications > Utilities > AirPort Utility
Click Manual Setup
Click the Internet icon, then the Internet Connection tab
The setting for Connection Sharing is likely Bridge Mode.
It should be Share a public IP address.
Make the change and click Update
Then, it would be a good idea to restart the entire network in sequence. Power everything off then start the Modem first, then next device, etc.
I still had problems, though going through your steps may have helped....like getting the setting out of Bridge Mode.
But in the end I had to start from scratch and make a new network. Both iMac and laptop are connected simultaneously now, though I haven't turned either of them off and rebooted which I guess will be the real test. But I'm hoping this is now resolved.
Thanks for your help.