How to extend Wi-Fi range and coverage?

My Belkin Wi-Fi N router will only cover half my house. Its located in the bedroom, the sitting room is on the other side of the house. I don't get a single bar on my iPhone or laptop. Usually over there I have to use expensive 3G to access mail etc. How do I increase its range and how do why do Wi-Fi N routers advertise the coverage when it doesn't even cover a normal sized house?

Wi-Fi repeater

Go to IP 192.168.1.1
it will ask for id and password type admin in both the fields as it is standard for all modems and networks.
then a menu will appear which is according to the manufacture of the modem there you will found a network button press it and then try to find the wireless setting or wireless wizard or something like that as i mentioned will be according to the modem provider.
there will be an option of the channel width increase it by the available option and your modem will deliver a more wide and good signal strength.
Hope it will help you out and if it did not helped you then you have purchase a better quality modem that will suite your requirement

IF possible, move the wi-fi router to the center of the house && as high as possible.
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House building materials, furniture, appliances, even plumbing/electric wiring block wi-fi signals.