1. Connect with the modem only.
Run the "ipconfig /all" command in a CMD window.
Write down the IP addresses, DNS addresses, Gateway address.
2. Attach the wireless router.
Access the router configuration page and check the addresses the router was given by the modem.
(User manual from the manufacturer might help.)
Run the "ipconfig /all" command on your computer in a CMD window.
Check the addresses the router gave your computer.
OR 3. Disable the router features in the wireless router. Use it as an access point.
I did this for my LAN.