What switch model number?
Make sure you are using the latest firmware for both your router and switch.

Here's how it should be set up:
Intenet --- modem --- wireless router --- switch

For the cable that connects the router and the switch, what port is it attached to on the router? On the switch?

Router has an IP address of ?
Is DHCP enabled on the router?

Did you give the switch a *static* IP address that is different from the router's IP address? Make sure IP address of the switch is not shared with any other network device (computers included).