Cable modem

I recently tried the netgear cg300d cable modem router. I was trying to upgrade to docsis 3. I currently use a motorola cable modem which is at docsis 2. I have a linksys wireless router. I bought the netgear online and had a bad experience. I couldn't get the throughput as advertised. I tried netgear support and it turned out to be in india. no go. I called cox and they wouldn't support the netgear becasue i didnt buy it from them. I returned the netgear and went back to my original setup. 

I would like to be able to use a a cable modem router or a newer cable modem that is at docsis 3. There are a number of them on the market although cox only supports a couple of them. 

Any advice is welcome.