Consistent Lagging during multiplayer/disconnecting since switching to New Xfinity modem

I cannot game at all with the consistent lagging and disconnecting from any multiplayer game I play since my own modem crapped out on me unfortunately this week. However, my motorola DOCSIS 3.0 modem capable of up to 900mbps on my current 1000mbps internet plan never, ever, lagged like this. I have a netgear nighthawk wireless router paired with the setup. 

Since switching to the xFi modem/router, my discord voice comes through sounding like a robot due to poor service(What friends are saying). Everything is business as usual so the only change has been going to this Xfinity modem/router that was originally sent to me after starting service to our new address when we moved. Spent 3 hours on the phone and Chat with Tech support trying to get answers, but no one can help me but tell me to reset my modem. I know all the tricks and troubleshooting methods and have already tried them and have done everything on my end to stop this from happening. What do I do? I pay $15.99 a month, to rent this. Is this what I am to expect from Xfinity moving forward? I'm really not trying to shift companies, but I'm a gamer in my free time (very little enough nowadays). I will do what I must but I'm trying to give you a last chance. On top of all of this, my sales person said my plan would not increase and that I would have the modem covered for 2 years free with my contract. Supposed to be paying $60 a month everything included first two years. I'm only 5 months in now and my bill and autopay say different every single month..........

Very much at the end of my rope......