Follow these steps in sequence:
1. Shut all PCs down.
2. Unplug the router.
3. Wait a minute or two.
4. Plug the modem back in and wait until all lights are green.
5. Reboot the PCs, one at a time.
Even if you have the correct IP addresses, it's not a bad idea to reboot everything occasionally. So if your Internet speeds are slow on all connections, or if you suffer from a loss of Internet connectivity, reboot the modem, router and PC for a quick fix.