Signal Booster for Samsung Galaxy S5

I recently purchased the Samsung Galaxy S5. The signal inside my apartment is very poor. My phone provider is T-Mobile while my internet provider is AT&T.

The T-Mobile employee gave my a Wi-FiCell Spot Router to boost the signal inside the apartment. The problem is that I don't know how to connect it to the AT&T equipment, that is a modem model 3800HGV-B and a wireless access point model VEN501.