[MovedGoalPosts]

Before you worry too much about the router settings, have a good look at the basic internet connection.

Connect a computer direct to the modem, not via the router. What do your speed tests now say? If you connect the xbox direct to the modem, again does the service improve? If there is a difference and the service with the router as your network reduces things, then you know something is amis with your router. If it doesn't improve, then perhaps the cable service is a little poor.

Try things like speedtest at various times of day as it can differ depending on what other traffic over the internet is taking place.