Only Some Websites Not Working On All Devices

I am having issue with some websites not working. It doesn’t matter what device I am using. When I am connected to the WiFi it won’t load specific websites. But when I am on another net work it works fine. I have this issue with 6 different devices. I counted the Internet provider (Cox) and they said everything looks good on their end and must be the modem. The modem is coming up on 4 years of age. Haven’t had any issues with it. This shouldn’t have any effect on it but the modem was unplugged for a week and once it has been reconnected it hasn’t been working properly. PLEASE HELP.