Need WAP for WNDR3400

Here's my dilemma. I live in a large single story house with an office in the center that I swear must have been a bomb shelter (the walls are solid concrete) in which wireless signals become severely degraded. To the East of that office are three rooms with devices in each that need a wireless connection (one can be wired).

No wireless router I have purchased (but for one) can penetrate the office walls and provide a strong signal.

I currently have a WNDR3400 Dual Band router that cannot penetrate the walls. I have tried using a Linksys WRT54G router as a WAP with both stock firmware and DD WRT, but the thing's Internet connection was too unstable.

Is there a Netgear router that can be easily turned into a WAP and be connected to my WNDR3400 via LAN without the use of DD WRT?

I have tried the range extenders but they reduce Internet speed by up to 75% in my case. All three of the bedrooms, including the office, need to have a stable and strong (10mbps minimum) connection.

Any and all help would be much appreciated! My next step is to try and run ethernet cable around the house but I'd like to avoid that :( !