Plan to establish a permanent Ethernet cable connection from one of the available LAN <-> ports on your modem/router to the WAN "O" port on the AirPort Extreme. Locate the AirPort Exteme in a central location in the general area where more wireless coverage is desired.
Configure the AirPort Extreme using AirPort Utility to "create" a wireless network using the exact same wireless network name, same exact wireless security settings and same exact password that your modem/router uses.
Finally, the Extreme must be configured in Bridge Mode to operate correctly with the modem/router.
To get everything in the proper sequence on the network....once the Extreme is configured, power off the entire network for a few minutes. Then start the modem/router first and let it run a minute by itself. Then start the Exteme next the same way. Continue starting devices the same way until the entire network is back up.