Netgear cm600 speed drop after connecting to router

Hi all,

I am with Comcast for 100Mbps plan.

My internet speed is harboring around 10Mbps. I contacted Comcast online tech support 4 times. For the first 3 times, they were able to troubleshoot by bringing the speed back to 100Mbps.

I have older Linksys router, ES3000. After connecting this router to Netgear CM600, I don't not have internet connection. I have to do the "MAC address clone" suggested by the following page. I thought it was a "plug and play".

http://www.linksys.com/us/support-article?articleNum=142912

Now, I have internet connection but the speed drops back down to 10Mbps. Even though I am connecting directly to Netgear CM600 via hard wired (bypassing the router), the speed will not go up. I have to contact Comcast tech support to bring it up to 100Mbps.

Here is my dilemma, I can only get 100 Mbps from Netgear CM600 by connecting directly without the router.

What did I do wrong?

Any idea will be greatly appointed. Thanks in advance.