WiFi Strength and Ubee DVW326

I have a Ubee DVW 326 from cox.  If I am next to the modem, wireless is fine, as soon as I leave the room the wifi drops in and out constantly.  This happens on any wifi device.  I see that the modem is only single band, could this be the issue.  Is there a range extender that works best with this modem?  Would I be better off exchanging for a dual band device?  About at my wits end.  Any advice would be appreciated.