Internet Speed and Data Usage

I'm having trouble with my internet, it's slow, intermittent and unreliable. I'm also being told that I have used almost 700 gigs of data usage my very first month with Cox, how is that even humanly possible? Something doesn't add up and makes no sense... I also did some research and found out that the Modem I purchased from Cox for almost 160 dollars is a less than par modem to say the least, in fact it's being sold brand new on Amazon for 79 bucks... what the HECK? I feel like I just got ripped off, have slow and unreliable internet and I'm being told I use to much bandwidth. NEVER in my life have I used so much bandwidth, in fact, Cox is the one who set it all up and password protected my wireless network, so I know it's not being stolen by the neighbors. I really feel like a fool for falling for something that seemed almost too good to be true. I just want internet that is reliable and works and I want to know that I'm not being taken advantage of, because my regular speed tests certainly show that I was and the cheap ol router I paid double for isn't giving me what I hoped. I'd love to have someone from Cox contact me so I can get these issues resolved please! Thanks