i have netgear 3500l and i have one huge problem

when i go to my other room my signal is only 1 bar :(

i tried to change mw to 200 but still same signal :(

i use b/g because i use my iphone and samsung pixon for wifi on my other room to check my emails etc.

any ideas how i can improve my signal my router pass between 2 walls and my bathroom i live in flat with beton+metal walls :(

i wait for your suggestions......

may be i must make some kind of antena or something ?

 I also have tp-link router.Which i dont use at the moment because i replace it with netgear 3500l but with tp-link i have same signal.