Slow Internet speeds of ~3 Mbps when I'm paying for 500 Mbps

Hello!

Like the title says, I'm dealing with two orders of magnitude slower Internet speed than I was expecting. According to Xfinity's own speed test, I'm pulling ~3 Mbps download speeds:

I am using my own modem and router, so I'm fully willing to admit that this might be my fault and not Xfinity's fault, but I want to get to the bottom of it regardless. A network diagram:

```none
Coaxial outlet in my apartment <--------> Modem <---------> Router <---------> My laptop
                                                        A                           B                         C
```

where
- A = an Amphenol TFC-T10 coaxial cable
- Modem = Netgear CM1200
- B = a CAT 5E Ethernet cable
- Router = Netgear R6700v3 Nighthawk running Fresh Tomato

- C = WiFi

Note that I have tried multiple Ethernet cables, so I am confident that the problem does not exist with B, the connection between the modem and the router. I have also tried to unplug / reboot the modem and router both multiple times. I'm also confident that the issue is not with C, connecting to the router over WiFi, as my router reports the same ballpark bandwidth that I see on my laphttps://https://prod-care-community-cdn.sprinklr.com/community/cd58a8fd-ee2e-46c9-8c79-e9309441066e/public-assets/" width="961">

To my understanding, the issue must be either:

1) A defect in my modem
2) A defect in the coaxial cable I am using to connect my modem to my outlet
3) An issue on Xfinity's end of things

I'm looking for help resolving this issue so I can get speeds of at least 100 Mbps, within an order of magnitude of the service I am paying for. Some more diagnostic info, here are the metrics for my modem at the time I'm writing this post:

Thanks for taking the time to help me with this issue!