AT&T fiber - Using ASUS RT-AC68U with BGW210

I recently had installed ATT&T fiber. I received the modem/router BGW210. The signal is really bad. How can I used my own router that I had before with my ex-ISP. I have an ASUS RT-AC68U. The tech that did the installation told me I couldn't use the router because the equipment install is modem/router. Is there a way of disable the router portion and just use it as a modem? Please help me. My room is only above my living room where the modem/router is and I didn't have this issue before.