That solution doesn't work anymore. There are too many client devices nowadays, and they have very small antennas (smartphones). You mentioned your building is concrete and covered in mirrors... this is probably the worst environment for WiFi (concrete = difficult to penetrate, causing signal loss; mirrors = very reflective, causing interference)
The best (maybe only) way to go about it today is to install an access point in every room, or in every other room. Look at the H320 or H510 APs. If there is no network cabling in the rooms and it is too expensive to cable, look at the C110.
Which country is this in?