Wireless Internet Range Question

I moved into my new house this week and had AT&T come out and install our internet and cable. Our internet works great however in our bedroom the signal is weak and goes in and out. I have 2 extra belkin N600 routers. I asked the tech if I could install one in our bedroom to extend the range and he said we would have to use an ATT router. Is this correct? If not, how do I hook up my routers to extend the range of our internet. I also bought a hard wire extender where I plug a box into the wall and run Ethernet cable to it then I can plug another box into another plug in and run Ethernet to that box and have hardwire internet.

Tia.