Netgear and Charter

I have both a Netgear modem (CM600) and router (X10) with Charter hi-speed internet service, and have no issues with the Netgear equipment.  

Here is a list of Charter's supported modems: 

http://www.spectrum.net/support/internet/compliant-modems-charter-network/

The only thing I can think of -without questioning the tech's conclusion- is that yours is not on the supported list of cable modems.

You might want to explain in greater detail what the issue is, your cable modem and router models, and what the tech did resolve this before coming to his conclusion.  Also, you can now get a free cable modem from Charter as long as you have their hi-speed internet service. The expertise of Charter's subcontractors vary wildly, BTW, so just be aware of that. If your cable modem and router aren't out of scope and functioning normally, call Charter back and have someone else come back out and check the outside connections, coax cables and whatever else falls under their purview.

 Also, as part of your troubleshooting process to eliminate your router as the issue, just connect your computer or laptop with an Ethernet cable directly to your cable modem, thereby bypassing your router. If you're connecting to the internet at that point, you have some troubleshooting to do on the router.