Connect the computer to the router with an ethernet cable.
See the literature which came with your router for how to access the user setup screens in the router. Once in go to wireless security and review or renew the wireless security type and password. WPA PSK TKIP is the most common security type in use but check that you pick one compatible with both the printer and your computer's wireless networking.
Go to wireless mode and ensure that it's set to a protocol matching your computer's wireless adapter and the printer's (mixed mode is a safe bet).
Apply the wireless security type and password to the printer's wireless networking setup screen.