Are you using at least a cat5e (or better) ethernet cord ?
Hello, I am getting around 90Mbps download speed on my xbox using ethernet and my plan is for 300Mbps. I am getting 140Mbps over wifi on my phone..what's going on? My xbox is only thing connected to the gateway at the moment.
First, test your speed with a computer by ethernet to the gateway, preferably with a dedicated app like the OOKLA Speedtest app.