If you are already setting up your own router, it seems like a huge waste to rent just the modem from them. Setting up your own cable modem requires little more than plugging it in, and your ISP should offer support if it's one of their approved modems.

With the current regulation regime, bringing your own modem on spectrum has meant that you give up the ability to get the highest performing tiers. This despite the fact that the modems on the open market were actually better/more capable than the spectrum only modem (aka not available publicly) they were giving people for the gigabit access. (this BTW just recently changed).

Even more fun, is that they regularly refuse to provision a modem at its full capability despite your service tier. AKA you have a modem that does X, and a tier that does X, but they will only provision it for 1/2X (and not bother to tell you that). A few years ago I fought a long protracted battle with their support/escalation/etc over a few months to get my modem qualified at the higher tiers and in the end the best excuse they gave was that it was a business decision.

AKA, making me upgrade my modem for no technical reason played into their ploy for renting me one.

What company was this? Couldn't you just fake your vendor to them somehow by spoofing your mac address?

I have a Comcast router modem combo(group contract through HOA, so free for me). My advertised is 125 Mbps and I do hit close to 125 when I use many of those speed test websites.

When I set this hardware in bridge mode and try to use my own router with dd-wrt, my speeds drop to below 50 Mbps.

I could never figure out why.

What speed did you get in bridge mode with a single PC directly connected to the modem? Perhaps your router was hitting some hardware performance limit when doing NAT and everything at full speed.

No different. The speed even with ethernet never go above 50-55 mbps.