Need help with Wifi Router

I bought a BEETEL 450TC1 ADSL2+ ROUTER a month back and using it with both desktop and laptop...but the problem is the wifi signal strength is just about 45% no matter where i put it....signal doesn't get stronger....what maybe the problem? Should I complain this to airtel guys??? My wifi connection is lost when i go to some other room and the room is in the range of 15 metersIf there is anything I can do in setting up the modem plz tell me...as I'm perfectly fine with changing my interface setting or tell me to just call the airtel and teach them a lesson..

Make sure your router is set to work in 20/40 Mhz mode.(This is a guess)