I would do a reset of the router. You'll have to re-enter in your PPPoE information. I presume you have DSL by the looks of it?
After that I would set up reserved static IP addresses for each device you connect to the router including wireless. Ensure DNS relay is enabled under Setup/Network Settings.
Ensure that the Primary and 2ndary DNS ip addresses are being filled in under the Setup/Internet/Manual. If there not then you can find them at the Status/Device Info/Wan section. Copy and paste them into there boxes respectively.
If you don't use WiFi modes A, B or sometime N, I would set the Wireless mode for G only. Its no use broadcasting modes that your not using.
You can also try and see if changing the Firewall settings to End Point Independent for both TCP and UDP.
If it works, save off the configuration to file for later use should you have to reset the router.
Let us know how it goes.Router Troubleshooting Suggestions and Tips