What ISP service do you have? DSL or cable?
What ISP modem do you have? Stand alone or built in router?
If DSL, it's recommended to bridge the DSL modem. Also If DSL, make sure the routers PPPoE information is correct and that ALWAYS ON is enabled.
I would try this, do a factory reset of the router.
Set all connected devices to static Reserved IP addresses on the router.
Make sure all other devices are auto obtain IP addressing.
Set WiFi mode from Mixed to Single mode G or Mixed G and N if you have any N supported devices.
Ensure your Primary and 2ndary DNS IP addresess are auto filled in under Setup/Internet/Manual.
Ensure DNS relay is enabled.
Disable Advanced DNS Services if you have this option.
Disable WISH if you have this option.
Disable uPNP under Advanced/Advanced network. Try with it disabled and enabled. See what happens.
Let us know how it goes.