server error with Netgear model WNDR4500v2

Certain websites give me an error - cannot connect to server. I have had my internet provider out adn was told it was not them. I updated my Apple Mac to the Sierra version which able me to sign on to  websites and pay bills which I wasn't able to do before. Apple support suggested updating router. Tried to do that and still get the server error message. VERY FRUSTRATRED!! This has been going on for months.  Certain websites I am not able to download purchases because of the error.  Other websites I am not able to view

Can someone please assist me?