Network extenders

Back in December, I called because I was having slow internet issues.  The tech came out and said it was because my modem was too old (~5 years) and installed a Cisco DPQ3925 Gateway.  The range on it is terrible and fluctuates in intensity, so I'd like to install a network extender but I've gone through two now with no success in being able to get the extenders to "talk" with the gateway.  

I already had a Hawking Technology ext that was great, but after failed attempts, I purchased a Netgear "universal" one that the head guy of networking at the (very reputable) store said should work with anything.  Alas, he was wrong.  Cisco says that all they do is build them and send to Cox, who programs them and they can't help me.  

Do I need a specific brand of extender to get this to happen?  

If not, will either of the Hawking or Netgear ones work if I dig into the programming of the Gateway to get it to connect/talk to the extender(s)?

Thanks!