I was on the phone today with Linksys elevated support for more than 2 hours.
Here is the bottom line so maybe you won't have to suffer as I have.
Step 1: Upgrade the firmware. In my case, I went from whatever to 1.1.7.160.177.
Step 2: Under Connectivity > Internet Settings, enable "MAC Address Clone".
Step 3: Click "Clone My PC's MAC".
Step 4: Click "Apply"
Done. Problem solved.
PS--My ISP is Cox Communications.