Why are my speeds so slow?

I’m supposed to be getting 400mbps and I ran a speed test on spectrums website standing less than a foot away from my modem and router and my speeds have never jumped past 55mbps. I have an Arris tm1602a docsis 3.0 modem that spectrum provided about 7 years ago when I first purchased them. I have moved 3 times and they have never told me I needed to upgrade my modem. What could be causing my low speeds?