Was curious if your ISP modem that the router is connected to if it has a built in router as well as the modem part or if it's just a stand alone modem with out any built in router features. If it has a built in router, this can cause connection issues having 2 routers on the same line and network.
What is the brand name and model number of the ISP modem?
You are wired to the router from the PC, not connecting wireless?
Is the Wireless radio on and running? Are you using it?
You can set the WiFi mode from Mixed to Single mode G or Mixed G and N if you have N supporting devices.