2 years with 400 to 500 speeds? I would call them up and ask for a downgrade to the 400 mbps service. In my case, it's overprovisioned anyway to around 500, and sometimes I will clock even higher speeds. At the very least you'll be paying them for what you're getting.
We need some details of your setup...what devices, how are they connected, what speed test are you using, how is the modem connected to the main line (splitters, length of coax run, etc), what to tests show when connected directly to the modem?
What provider does your neighbor have?
Have you direct connected to the modem? Are you in an apartment building or a house? What is the coax setup behind the wall back to the tap? Has a tech tested directly with their meter and what was their finding?
If you are paying for "Gigabit" internet connection, then Spectrum should be able to measure 930 Mbps at any time. When Speedtest reports 600 Mbps while your computer is connected directly to the modem, then that's the speed Spectrum is delivering at that time. Any decrease from that number when running through your router or WiFi demonstrates a router or WiFi problem.
I've worked on several large scale networks. So, I understand how infrastructure, equipment, packets, sunspots, solar flares, etc... can interfere. But, if if your'e paying for One Gbps. I would hope you get as close as possible to that speed. When I pay for my food, or a car, or a house. I get exactly what I pay for. So, why should I need to worry about sharing bandwidth to a server. Or who I should run my speed test against.. Obviously, your technicians use equipment with preset hosts to test against. They also test that the physical connection can handle the speeds reliably. But, why always only 400 - 500? It seems like everyone has that same exact limitation? When I first had mine installed, it was around 700 - 800. Now, it's barely 500....ALL THE TIME! Shouldn't it fluctuate? There is NEVER a time it get's close to 700 or 800. This is all while DIRECTLY wired to the modem provided by Spectrum. Go figure.