arris sbg6580 - replace or something else

I have had this sbg6580 for years now and has been good except range.  We had an extender that helped but when the painter came they lost it.  So, I was looking to replace the extender and called COX. I did a speed test while I was at it and even though COX had download speeds up to 150 mbps. I am getting 50.  Also, I cannot get wifi reception even 30 feet down the all.    As for the speed, COX is saying modem. I doubt that. So, now I am trying to figure out should I just get extender(s) or should I get a new modem/router and possibly extender. The house is only 2500 square feet (2 stories).   

Any assistance/advice appreciated.