You have a few options:
1. Some routers have a signal strength adjustment in the software. You would have to log into the router as the administrator.
2. If it is possible you could relocate the router closer to the lanai. (more centrally located in the house)
3. You can go to a store (like best buy) and buy a Wi-Fi repeater (Wi-Fi Range Extender). (cost about 60 dollars)
NETGEAR Universal WiFi Range Extender with 4port Ethernet Switch WN2000RPT-100NAS - Best Buy